How can I convert a string to boolean in JavaScript

Question

Can I convert a string representing a boolean value  e g    true    false   into a intrinsic type in JavaScript   I have a hidden form in HTML that is updated based upon a user s selection within a list  This form contains some fields which represent boolean values and are dynamically populated with an intrinsic boolean value  However  once this value is placed into the hidden input field it becomes a string   The only way I could find to determine the field s boolean value  once it was converted into a string  was to depend upon the literal value of its string representation   var myValue   document myForm IS TRUE value  var isTrueSet   myValue     true     Is there a better way to accomplish this

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Building on Steven s answer above  I wrote this function as a generic parser for string input   parse    function  value        switch  value  amp  amp  value toLowerCase            case null  return null        case  true   return true        case  false   return false        default  try   return parseFloat value     catch  e    return value

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If there s some other code that s converting the boolean value to a string  you need to know exactly how that code stores true false values  Either that or you need to have access to a function that reverses that conversion   There are infinitely many ways to represent boolean values in strings   true    Y    1   etc    So you shouldn t rely on some general-purpose string-to-boolean converter  like Boolean myValue   You need to use a routine that reverses the original boolean-to-string conversion  whatever that is   If you know that it converts true booleans to  true  strings  then your sample code is fine  Except that you should use     instead of     so there s no automatic type conversion

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I ve been using this snippet to convert Numbers and Booleans   var result    isNaN value    parseFloat value       s  true false  s   i exec value    RegExp  1 toLowerCase        true    value

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WARNING  Never use this method for untrusted input  such as URL parameters  You can use the eval   function  Directly pass your string to eval   function   x000D   x000D  console log eval  true    typeof eval  true    console log eval  false    typeof eval  false    x000D   x000D   x000D

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To convert both string  true    false   and boolean to boolean        flag       true    Where flag can be    var flag   true  var flag    true   var flag   false  var flag    false

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If you are certain that the test subject is always a string  then explicitly checking that it equals true is your best bet   You may want to consider including an extra bit of code just in case the subject could actually a boolean   var isTrueSet       myValue     true        myValue    null  amp  amp      myValue toString   toLowerCase        true     This could save you a bit of work in the future if the code gets improved refactored to use actual boolean values instead of strings

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I hope this is a most comprehensive use case  function parseBoolean token      if  typeof token      string         switch  token toLowerCase            case  on         case  yes         case  ok         case  ja         case                case              case             token   true          break        default          token   false              let ret   false    try       ret   Boolean JSON parse token        catch  e           do nothing or make a notification       return ret

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Remember to match case   var isTrueSet    myValue toLowerCase        true      Also  if it s a form element checkbox  you can also detect if the checkbox is checked   var isTrueSet   document myForm IS TRUE checked    Assuming that if it is checked  it is  set  equal to true  This evaluates as true false

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To convert both string  true    false   and boolean to boolean        flag       true    Where flag can be    var flag   true  var flag    true   var flag   false  var flag    false

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function isTrue val        try           return   JSON parse val         catch  e            return false

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I m a little late  but I have a little snippet to do this  it essentially maintains all of JScripts truthey falsey filthy-ness but includes  false  as an acceptible value for false   I prefer this method to the ones mentioned because it doesn t rely on a 3rd party to parse the code  i e  eval JSON parse   which is overkill in my mind  it s short enough to not require a utility function and maintains other truthey falsey conventions   var value    false   var result    value     false      Boolean value       value    true     gt  result   true    value    false    gt  result   false    value   true      gt  result   true    value   false     gt  result   false    value   null      gt  result   false    value             gt  result   true    etc

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There are a lot of answers and it s hard to pick one  In my case  I prioritise the performance when choosing  so I create this jsPerf that I hope can throw some light here   Brief of results  the higher the better     Conditional statement  2 826 922 Switch case on Bool object  2 825 469 Casting to JSON  1 867 774    conversions  805 322 Prototype of String  713 637   They are linked to the related answer where you can find more information  pros and cons  about each one  specially in the comments

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Do   var isTrueSet    myValue     true      You could make it stricter by using the identity operator        which doesn t make any implicit type conversions when the compared variables have different types  instead of the equality operator        var isTrueSet    myValue      true        Don t   You should probably be cautious about using these two methods for your specific needs   var myBool   Boolean  false           true  var myBool      false          true   Any string which isn t the empty string will evaluate to true by using them  Although they re the cleanest methods I can think of concerning to boolean conversion  I think they re not what you re looking for

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function isTrue val        try           return   JSON parse val         catch  e            return false

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The simplest way to convert a string to a boolean is the following  Boolean  lt stringVariable gt

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Take care  maybe in the future the code change and return boolean instead of one string at the moment   The solution would be     Currently var isTrue    true     In the future  Other developer change the code  var isTrue   true    The solution to both cases  isTrue  toString       true

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Your solution is fine   Using     would just be silly in this case  as the field s value will always be a String

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Try this solution  it works like a charm     function convertStrToBool str                switch String str  toLowerCase                                  case  undefined   case  null   case  nan   case  false   case  no   case  f   case  n   case  0   case  off   case                         return false                      break                  default                      return true

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I think this is much universal  if  String a  toLowerCase       quot true quot       It goes  String true      quot true quot        returns true String false      quot true quot       returns false String  quot true quot       quot true quot      returns true String  quot false quot       quot true quot     returns false

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Simple one line operation if you need Boolean false and true from the string values  storeBooleanHere   stringVariable   quot true quot  true false    storeBooleanHere - This variable will hold the boolean value stringVariable - Variable that has boolean stored as string

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The fastest safe way to convert a string to a boolean in one line of code One of features that help to fasten the code execution in Javascript is Short-Circuit Evaluation   As logical expressions are evaluated left to right  they are tested for possible  quot short-circuit quot  evaluation using the following rules   false  amp  amp   anything  is short-circuit evaluated to false  true     anything  is short-circuit evaluated to true    So that if you want to test a string value for being true of false in JSON parse way of test and keep the performance strong  you may use the    operator to exclude the slow code from execution in case the test value is of boolean type  test     true      true   yes   1   indexOf test toString   toLowerCase     gt  -1  As the Array prototype indexOf   method is a part of ECMA-262 standard in the 5th edition  you may need a polyfill for the old browsers support     Production steps of ECMA-262  Edition 5  15 4 4 14    Reference  http   es5 github io  x15 4 4 14 if   Array prototype indexOf      Array prototype indexOf   function searchElement  fromIndex         var k          1  Let O be the result of calling ToObject passing           the this value as the argument      if  this    null          throw new TypeError   quot this quot  is null or not defined               var O   Object this           2  Let lenValue be the result of calling the Get           internal method of O with the argument  quot length quot          3  Let len be ToUint32 lenValue       var len   O length  gt  gt  gt  0          4  If len is 0  return -1      if  len     0          return -1                5  If argument fromIndex was passed let n be           ToInteger fromIndex   else let n be 0      var n    fromIndex    0       if  Math abs n      Infinity          n   0                6  If n  gt   len  return -1      if  n  gt   len          return -1                7  If n  gt   0  then Let k be n         8  Else  n lt 0  Let k be len - abs n             If k is less than 0  then let k be 0      k   Math max n  gt   0   n   len - Math abs n   0           9  Repeat  while k  lt  len     while  k  lt  len             a  Let Pk be ToString k              This is implicit for LHS operands of the in operator          b  Let kPresent be the result of calling the             HasProperty internal method of O with argument Pk             This step can be combined with c          c  If kPresent is true  then             i   Let elementK be the result of calling the Get                 internal method of O with the argument ToString k              ii   Let same be the result of applying the                 Strict Equality Comparison Algorithm to                 searchElement and elementK            iii   If same is true  return k        if  k in O  amp  amp  O k      searchElement            return k                k              return -1

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For TypeScript we can use the function   export function stringToBoolean s  string  valueDefault  boolean   false   boolean       switch s toLowerCase                  case  true           case  1           case  on           case  yes           case  y               return true           case  false           case  0           case  off           case  no           case  n               return false             return valueDefault

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You need to separate  in your thinking  the value of your selections and the representation of that value    Pick a point in the JavaScript logic where they need to transition from string sentinels to native type and do a comparison there  preferably where it only gets done once for each value that needs to be converted  Remember to address what needs to happen if the string sentinel is not one the script knows  i e  do you default to true or to false    In other words  yes  you need to depend on the string s value   -

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Warning  This highly upvoted legacy answer is technically correct but only covers a very specific scenario  when your string value is EXACTLY  true  or  false    An invalid json string passed into these functions below WILL throw an exception     Original answer   How about   JSON parse  True  toLowerCase       or with jQuery    parseJSON  TRUE  toLowerCase

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guinaps  Any string which isn t the empty string will evaluate to true by using them   How about using the String match   method  var str  true   var boolStr Boolean str match   true  i       this alone won t get the 1 0 or the yes no  but it will catch the TRUE true  as well  it will return false for any string that happens to have   true  as a substring   EDIT  Below is a function to handle true false  1 0  yes no  case-insensitive    function stringToBool str        var bool      if  str match    true 1 yes   i      null            bool   true        else if  str match    false 0 no    i      null            bool   false        else           bool   null          if  console  console log       str      is not a boolean value              return bool     stringToBool  1       true stringToBool  No       false stringToBool  falsey       null   falsey  is not a boolean value   stringToBool         false

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Building on Steven s answer above  I wrote this function as a generic parser for string input   parse    function  value        switch  value  amp  amp  value toLowerCase            case null  return null        case  true   return true        case  false   return false        default  try   return parseFloat value     catch  e    return value

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I m using this one  String prototype maybeBool   function         if     yes    true    1    on   indexOf  this toLowerCase         -1   return true      if     no    false    0    off   indexOf  this toLowerCase         -1   return false       return this       on  maybeBool      returns true   off  maybeBool      returns false   I like js  maybeBool      returns  I like js

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I use the following   function parseBool b        return      false 0   i  test b   amp  amp    b      This function performs the usual Boolean coercion with the exception of the strings  false   case insensitive  and  0

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I think this is much universal  if  String a  toLowerCase       quot true quot       It goes  String true      quot true quot        returns true String false      quot true quot       returns false String  quot true quot       quot true quot      returns true String  quot false quot       quot true quot     returns false

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String true  toLowerCase       true      true String  true   toLowerCase       true      true String  True   toLowerCase       true      true String  TRUE   toLowerCase       true      true  String false  toLowerCase       true      false   If you are not sure of the input  the above works for boolean and as well any string

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You even do not need to convert the string to boolean  just use the following  var yourstring   yourstringValue     1   true   false

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Take it easy using this lib   https   github com rohmanhm force-boolean  you just need to write a single line  const ForceBoolean   require  force-boolean    const YOUR VAR    false  console log ForceBoolean YOUR VAR      it s return boolean false   It s also support for following   return false if value is number 0  return false if value is string  0   return false if value is string  false   return false if value is boolean false  return true if value is number 1  return true if value is string  1   return true if value is string  true   return true if value is boolean true

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Simple solution i have been using it for a while  function asBoolean value         return     value       true            asBoolean true     gt  true    asBoolean false     gt  false    asBoolean  true      gt  true    asBoolean  false      gt  false

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Universal solution with JSON parse   function getBool val        return   JSON parse String val  toLowerCase        getBool  1      true getBool  0      false getBool  true      true getBool  false      false getBool  TRUE      true getBool  FALSE      false   UPDATE  without JSON    function getBool val        var num    val      return  isNaN num      num     String val  toLowerCase   replace   0          I also created fiddle to test it http   jsfiddle net remunda 2GRhG

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This has been taken from the accepted answer  but really it has a very weak point  and I am shocked how it got that count of upvotes  the problem with it that you have to consider the case of the string because this is case sensitive  var isTrueSet    myValue toLowerCase        true

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I think this is much universal  if  String a  toLowerCase       quot true quot       It goes  String true      quot true quot        returns true String false      quot true quot       returns false String  quot true quot       quot true quot      returns true String  quot false quot       quot true quot     returns false

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Holy god some of these answers are just wild   I love JS and its infinite number of ways to skin a bool   My preference  which I was shocked not to see already  is   testVar   testVar toString   match    true  1-9  0-9    0-9   1-9   yes   i    true   false

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A lot of the existing answers are similar  but most ignore the fact that the given argument could also be an object   Here is something I just whipped up   Utils parseBoolean   function val       if  typeof val      string     val instanceof String           return  true i test val         else if  typeof val      boolean     val instanceof Boolean           return new Boolean val  valueOf          else if  typeof val      number     val instanceof Number           return new Number val  valueOf       0            return false          and the unit test for it  Utils Tests   function        window console log  running unit tests         var booleanTests               true   true             false   false             True   true             False   false              false            true  true            false  false             gibberish   false            0  false            1  true              for  var i   0  i  lt  booleanTests length  i             var lhs   Utils parseBoolean booleanTests i  0            var rhs   booleanTests i  1           var result   lhs     rhs           if  result               console log  Utils parseBoolean   booleanTests i  0           booleanTests i  1    t    tpass              else               console log  Utils parseBoolean   booleanTests i  0           booleanTests i  1    t    tfail

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I wrote a function to match PHP s filter var which does this nicely  Available in a gist  https   gist github com CMCDragonkai 7389368         Parses mixed type values into booleans  This is the same function as filter var in PHP using boolean validation     param   Mixed         value      param   Boolean       nullOnFailure   false     return  Boolean Null      var parseBooleanStyle   function value  nullOnFailure   false       switch value           case true          case  true           case 1          case  1           case  on           case  yes               value   true              break          case false          case  false           case 0          case  0           case  off           case  no               value   false              break          default              if nullOnFailure                   value   null               else                  value   false                            break            return value

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var falsy        f   alse   no  0    i  Boolean parse   function val         return  falsy test val   amp  amp    val       This returns false for every falsy value and true for every truthy value except for  false    f    no    n   and  0   case-insensitive       False Boolean parse false   Boolean parse  false    Boolean parse  False    Boolean parse  FALSE    Boolean parse  f    Boolean parse  F    Boolean parse  no    Boolean parse  No    Boolean parse  NO    Boolean parse  n    Boolean parse  N    Boolean parse  0    Boolean parse      Boolean parse 0   Boolean parse null   Boolean parse undefined   Boolean parse NaN   Boolean parse       True Boolean parse true   Boolean parse  true    Boolean parse  True    Boolean parse  t    Boolean parse  yes    Boolean parse  YES    Boolean parse  y    Boolean parse  1    Boolean parse  foo    Boolean parse      Boolean parse 1   Boolean parse -1   Boolean parse new Date

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I use the following   function parseBool b        return      false 0   i  test b   amp  amp    b      This function performs the usual Boolean coercion with the exception of the strings  false   case insensitive  and  0

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I recommend you to create a function like the third option in the image and place it in a helper class as export  and reuse this function when you need

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The Boolean object doesn t have a  parse  method  Boolean  false   returns true  so that won t work     false  also returns true  so that won t work also   If you want string  true  to return boolean true and string  false  to return boolean false  then the simplest solution is to use eval    eval  true   returns true and eval  false   returns false  Keep in mind the performance implications when using eval   though

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Try this solution  it works like a charm     function convertStrToBool str                switch String str  toLowerCase                                  case  undefined   case  null   case  nan   case  false   case  no   case  f   case  n   case  0   case  off   case                         return false                      break                  default                      return true

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To evaluate both boolean and boolean-like strings like boolean I used this easy formula   var trueOrStringTrue    trueOrStringTrue     true      trueOrStringTrue      true      As is apparent  it will return true for both true and  true   Everything else returns false

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if you are sure the input is anything only within  true  and  false  why not    x000D   x000D  let x    true      let x   false   let y   x    true  true false  console log typeof y   y   x000D   x000D   x000D

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WARNING  Never use this method for untrusted input  such as URL parameters  You can use the eval   function  Directly pass your string to eval   function   x000D   x000D  console log eval  true    typeof eval  true    console log eval  false    typeof eval  false    x000D   x000D   x000D

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stringToBoolean  function string       switch string toLowerCase   trim             case  true   case  yes   case  1   return true          case  false   case  no   case  0   case null  return false          default  return Boolean string

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Many of the existing answers use an approach that is semantically similar to this  but I think there is value in mentioning that the following  one liner  is often sufficient  For example  in addition to the OP s case  strings in a form  one often wants to read environment variables from process env in NodeJS  whose values  to the best of my knowledge  are always strings  in order to enable or disable certain behaviors  and it is common for these to have the form SOME ENV VAR 1   const toBooleanSimple    input    gt       t    y    1   some truePrefix   gt  truePrefix     input 0  toLowerCase       A slightly more robust and expressive implementation might look like this          Converts strings to booleans in a manner that is less surprising    to the non-JS world  e g  returns true for  1    yes    True   etc     and false for  0    No    false   etc       param input     returns  boolean      function toBoolean input      if  typeof input      string         return Boolean input         const s   input toLowerCase      return   t    y    1   some prefix   gt  s startsWith prefix        A  jest  unit test for this might look like this   describe  toBoolean   function       const groups          inputs    y    Yes    true    1   true  1       expectedOutput  true            inputs    n    No    false    0   false  0       expectedOutput  false        for  let group of groups        for  let input of group inputs          it  should return   group expectedOutput  for   JSON stringify input     function             expect toBoolean input   toEqual group expectedOutput

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Like  Shadow2531 said  you can t just convert it directly  I d also suggest that you consider string inputs besides  true  and  false  that are  truthy  and  falsey  if your code is going to be reused used by others  This is what I use   function parseBoolean string      switch  String string  toLowerCase          case  true       case  1       case  yes       case  y         return true      case  false       case  0       case  no       case  n         return false      default          you could throw an error  but  undefined  seems a more logical reply       return undefined

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You need to separate  in your thinking  the value of your selections and the representation of that value    Pick a point in the JavaScript logic where they need to transition from string sentinels to native type and do a comparison there  preferably where it only gets done once for each value that needs to be converted  Remember to address what needs to happen if the string sentinel is not one the script knows  i e  do you default to true or to false    In other words  yes  you need to depend on the string s value   -

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if  String a      true        true block   else       false block

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I do this  which will handle 1 TRUE yes YES true  0 FALSE no NO false   BOOL false if  STRING    BOOL JSON parse STRING toLowerCase   replace  no   false   replace  yes   true       Replace STRING with the name of your string variable   If it s not null  a numerical value or one of these strings   true    TRUE    false    FALSE    yes    YES    no    NO  It will throw an error  intentionally

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const result  Boolean   strValue      true    true   false

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Universal solution with JSON parse   function getBool val        return   JSON parse String val  toLowerCase        getBool  1      true getBool  0      false getBool  true      true getBool  false      false getBool  TRUE      true getBool  FALSE      false   UPDATE  without JSON    function getBool val        var num    val      return  isNaN num      num     String val  toLowerCase   replace   0          I also created fiddle to test it http   jsfiddle net remunda 2GRhG

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The fastest safe way to convert a string to a boolean in one line of code One of features that help to fasten the code execution in Javascript is Short-Circuit Evaluation   As logical expressions are evaluated left to right  they are tested for possible  quot short-circuit quot  evaluation using the following rules   false  amp  amp   anything  is short-circuit evaluated to false  true     anything  is short-circuit evaluated to true    So that if you want to test a string value for being true of false in JSON parse way of test and keep the performance strong  you may use the    operator to exclude the slow code from execution in case the test value is of boolean type  test     true      true   yes   1   indexOf test toString   toLowerCase     gt  -1  As the Array prototype indexOf   method is a part of ECMA-262 standard in the 5th edition  you may need a polyfill for the old browsers support     Production steps of ECMA-262  Edition 5  15 4 4 14    Reference  http   es5 github io  x15 4 4 14 if   Array prototype indexOf      Array prototype indexOf   function searchElement  fromIndex         var k          1  Let O be the result of calling ToObject passing           the this value as the argument      if  this    null          throw new TypeError   quot this quot  is null or not defined               var O   Object this           2  Let lenValue be the result of calling the Get           internal method of O with the argument  quot length quot          3  Let len be ToUint32 lenValue       var len   O length  gt  gt  gt  0          4  If len is 0  return -1      if  len     0          return -1                5  If argument fromIndex was passed let n be           ToInteger fromIndex   else let n be 0      var n    fromIndex    0       if  Math abs n      Infinity          n   0                6  If n  gt   len  return -1      if  n  gt   len          return -1                7  If n  gt   0  then Let k be n         8  Else  n lt 0  Let k be len - abs n             If k is less than 0  then let k be 0      k   Math max n  gt   0   n   len - Math abs n   0           9  Repeat  while k  lt  len     while  k  lt  len             a  Let Pk be ToString k              This is implicit for LHS operands of the in operator          b  Let kPresent be the result of calling the             HasProperty internal method of O with argument Pk             This step can be combined with c          c  If kPresent is true  then             i   Let elementK be the result of calling the Get                 internal method of O with the argument ToString k              ii   Let same be the result of applying the                 Strict Equality Comparison Algorithm to                 searchElement and elementK            iii   If same is true  return k        if  k in O  amp  amp  O k      searchElement            return k                k              return -1

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works perfectly and very simple    var boolean    false   boolean    boolean      true       boolean   JSON parse boolean     or this way      to test it     x000D   x000D  var boolean    false   x000D  boolean    boolean      true    x000D   x000D    boolean   JSON parse boolean     or this way    x000D   x000D  if boolean    true   x000D      alert  boolean     boolean   x000D   else  x000D      alert  boolean     boolean   x000D    x000D   x000D   x000D

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why don t you try something like this  Boolean JSON parse  yourString toString    toLowerCase        It will return an error when some other text is given rather than true or false regardless of the case and it will capture the numbers also as      0- gt  false    any other number - gt  true

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The strongest way is the following because it also handle undefined case         true   true   false   false   myValue           true   true   false   false   undefined       gt  undefined        true   true   false   false    true        gt  true        true   true   false   false    false       gt  false

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Boolean parse   does exist in some browser implementations   It s definitely not universal  so if that s something that you need than you shouldn t use this method   But in Chrome  for example  I m using v21  it works just fine and as one would expect

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In HTML the values of attributes eventually become strings  To mitigate that in undesired situations you can have a function to conditionally parse them into values they represent in the JavaScript or any other programming langauge of interest   Following is an explanation to do it for reviving boolean type from the string type  but it can be further expanded into other data types too  like numbers  arrays or objects   In addition to that JSON parse has a revive parameter which is a function  It also can be used to achieve the same   Let s call a string looking like a boolean   true   a boolean string likewise we can call a string like a number   1   a number string  Then we can determine if a string is a boolean string   const isBooleanString    string    gt    true    false   some item   gt  item     string     After that we need to parse the boolean string as JSON by JSON parse method   JSON parse aBooleanString     However  any string that is not a boolean string  number string  or any stringified object or array  any invalid JSON  will cause the JSON parse method to throw a SyntaxError   So  you will need to know with what to call it  i e  if it is a boolean string  You can achieve this by writing a function that makes the above defiend boolean string check and call JSON parse   function parse string     return isBooleanString string    JSON parse string        string      One can further generalize the isBooleanString utility to have a more broader perspective on what qualifies as a boolean string by further parametrizing it to accept an optional array of accepted boolean strings   const isBooleanString    string  spec     true    false    True    False      gt  spec some item   gt  item     string

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Like  Shadow2531 said  you can t just convert it directly  I d also suggest that you consider string inputs besides  true  and  false  that are  truthy  and  falsey  if your code is going to be reused used by others  This is what I use   function parseBoolean string      switch  String string  toLowerCase          case  true       case  1       case  yes       case  y         return true      case  false       case  0       case  no       case  n         return false      default          you could throw an error  but  undefined  seems a more logical reply       return undefined

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Here is my 1 liner submission  I needed to evaluate a string and output  true if  true   false if  false  and a number if anything like  -12 35673    val    false    val     false  i test val    false       true  i test val    true   val 1   val 1   val

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Here is simple function that will do the trick      function convertStringToBool str           return   str      True       str      true      true false          This will give the following result  convertStringToBool  false     returns false convertStringToBool  true      returns true convertStringToBool  False      returns false convertStringToBool  True      returns true

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The simplest way which I always use   let value    true   let output   value      true

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function parseBool value        if  typeof value      boolean   return value       if  typeof value      number             return value     1   true   value     0   false   undefined             if  typeof value     string   return undefined       return value toLowerCase        true    true   false

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You need to separate  in your thinking  the value of your selections and the representation of that value    Pick a point in the JavaScript logic where they need to transition from string sentinels to native type and do a comparison there  preferably where it only gets done once for each value that needs to be converted  Remember to address what needs to happen if the string sentinel is not one the script knows  i e  do you default to true or to false    In other words  yes  you need to depend on the string s value   -

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To Get Boolean values from string or number Here is good solution   var boolValue   Boolean Number  0      var boolValue   Boolean Number  1       First will return false and second will return true

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There are already so many answers available  But following can be useful in some scenarios      One can specify all values against which you consider truthy var TRUTHY VALUES    true   true   1    function getBoolean a        return TRUTHY VALUES some function t            return t     a              This can be useful where one examples with non-boolean values   getBoolean  aa       false getBoolean false     false getBoolean  false      false  getBoolean  true       true getBoolean true      true getBoolean 1      true

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I use an own method which includes a check if the object exists first and a more intuitive conversion to boolean   function str2bool strvalue     return  strvalue  amp  amp  typeof strvalue     string      strvalue toLowerCase       true     strvalue     1      strvalue    true       The results are   var test     false var test2   null     false var test3    undefined      false var test4    true      true var test5    false      false var test6   true     true var test7   false     false var test8   1     true var test9   0     false var test10    1      true var test11    0      false   Fiddle  http   jsfiddle net av5xcj6s

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I ve been using this snippet to convert Numbers and Booleans   var result    isNaN value    parseFloat value       s  true false  s   i exec value    RegExp  1 toLowerCase        true    value

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To evaluate both boolean and boolean-like strings like boolean I used this easy formula   var trueOrStringTrue    trueOrStringTrue     true      trueOrStringTrue      true      As is apparent  it will return true for both true and  true   Everything else returns false

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If there s some other code that s converting the boolean value to a string  you need to know exactly how that code stores true false values  Either that or you need to have access to a function that reverses that conversion   There are infinitely many ways to represent boolean values in strings   true    Y    1   etc    So you shouldn t rely on some general-purpose string-to-boolean converter  like Boolean myValue   You need to use a routine that reverses the original boolean-to-string conversion  whatever that is   If you know that it converts true booleans to  true  strings  then your sample code is fine  Except that you should use     instead of     so there s no automatic type conversion

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MyLib Convert bool   function param             var res   String param  toLowerCase             return    Boolean res     res      false     res      0

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stringToBoolean  function string       switch string toLowerCase   trim             case  true   case  yes   case  1   return true          case  false   case  no   case  0   case null  return false          default  return Boolean string

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I use this simple approach  using  myVarToTest     var trueValuesRange     1   1   true   true    myVarToTest    trueValuesRange indexOf myVarToTest   gt   0

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function parseBool value        if  typeof value      boolean   return value       if  typeof value      number             return value     1   true   value     0   false   undefined             if  typeof value     string   return undefined       return value toLowerCase        true    true   false

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There are a lot of answers and it s hard to pick one  In my case  I prioritise the performance when choosing  so I create this jsPerf that I hope can throw some light here   Brief of results  the higher the better     Conditional statement  2 826 922 Switch case on Bool object  2 825 469 Casting to JSON  1 867 774    conversions  805 322 Prototype of String  713 637   They are linked to the related answer where you can find more information  pros and cons  about each one  specially in the comments

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The most simple way is   a    True   a     a  amp  amp    1    true   1  true  indexOf a toLowerCase     gt  -1

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You can use Function to return a Boolean value from string  true  or  false    x000D   x000D  const TRUE OR FALSE   str   gt  new Function  return   str       x000D   x000D  const  TRUE  FALSE      true    false    x000D   x000D  const  T  F     TRUE OR FALSE TRUE   TRUE OR FALSE FALSE    x000D   x000D  console log T  typeof T       true    boolean   x000D   x000D  console log F  typeof F       false    boolean   x000D   x000D   x000D

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var myBool   Boolean parse  true      or  var myBool   Boolean  true      or   var myBool      true     Not sure if the first one is IE specific

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I wrote a function to match PHP s filter var which does this nicely  Available in a gist  https   gist github com CMCDragonkai 7389368         Parses mixed type values into booleans  This is the same function as filter var in PHP using boolean validation     param   Mixed         value      param   Boolean       nullOnFailure   false     return  Boolean Null      var parseBooleanStyle   function value  nullOnFailure   false       switch value           case true          case  true           case 1          case  1           case  on           case  yes               value   true              break          case false          case  false           case 0          case  0           case  off           case  no               value   false              break          default              if nullOnFailure                   value   null               else                  value   false                            break            return value

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Convert something to boolean function toBoolean  o         if   null     o             let t   typeof o          if    quot undefined quot      typeof o                 if    quot string quot      t   return   o              o   o toLowerCase   trim                return  quot true quot      o     quot 1 quot      o                      return false     toBoolean false  -- gt  false toBoolean true  -- gt  true toBoolean  quot false quot   -- gt  false toBoolean  quot true quot   -- gt  true toBoolean  quot TRue quot   -- gt  true toBoolean  quot 1 quot   -- gt  true toBoolean  quot 0 quot   -- gt  false toBoolean 1  -- gt  true toBoolean 0  -- gt  false toBoolean 123 456  -- gt  true toBoolean 0 0  -- gt  false toBoolean  quot  quot   -- gt  false toBoolean null  -- gt  false toBoolean   -- gt  false

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I thought that  Steven  s answer was the best one  and took care of a lot more cases than if the incoming value was just a string  I wanted to extend it a bit and offer the following   function isTrue value       if  typeof value       string            value   value trim   toLowerCase              switch value           case true          case  true           case 1          case  1           case  on           case  yes               return true          default               return false            It s not necessary to cover all the false cases if you already know all of the true cases you d have to account for  You can pass anything into this method that could pass for a true value  or add others  it s pretty straightforward   and everything else would be considered false

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My take on this question is that it aims to satisfy three objectives    Return true false for truthy and falsey values  but also return true false for multiple string values that would be truthy or falsey if they were Booleans instead of strings  Second  provide a resilient interface so that values other than those specified will not fail  but rather return a default value Third  do all this with as little code as possible    The problem with using JSON is that it fails by causing a Javascript error  This solution is not resilient  though it satisfies 1 and 3    JSON parse  FALSE      fails   This solution is not concise enough   if value      TRUE     value      yes            return true      I am working on solving this exact problem for Typecast js  And the best solution to all three objectives is this one   return   true  i test v     It works for many cases  does not fail when values like    are passed in  and is very concise  Also it returns false as the default value rather than undefined or throwing an Error  which is more useful in loosely-typed Javascript development  Bravo to the other answers that suggested it

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You can use regular expressions         Converts a string to a bool        This conversion will         - match  true    on   or  1  as true      - ignore all white-space padding     - ignore capitalization  case            tRue     ON   and  1     will all evaluate as true         function strToBool s           will match one and only one of the string  true   1   or  on  rerardless        of capitalization and regardless off surrounding white-space             regex    s  true 1 on  s   i      return regex test s       If you like extending the String class you can do   String prototype bool   function         return strToBool this       alert  true  bool       For those  see the comments  that would like to extend the String object to get this but are worried about enumerability and are worried about clashing with other code that extends the String object   Object defineProperty String prototype   com example bool         get   function             return     true 1   i  test this             alert  true  com example bool      Won t work in older browsers of course and Firefox shows false while Opera  Chrome  Safari and IE show true  Bug 720760

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If you are certain that the test subject is always a string  then explicitly checking that it equals true is your best bet   You may want to consider including an extra bit of code just in case the subject could actually a boolean   var isTrueSet       myValue     true        myValue    null  amp  amp      myValue toString   toLowerCase        true     This could save you a bit of work in the future if the code gets improved refactored to use actual boolean values instead of strings

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var myBool   Boolean parse  true      or  var myBool   Boolean  true      or   var myBool      true     Not sure if the first one is IE specific

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Wood-eye be careful  After seeing the consequences after applying the top answer with 500  upvotes  I feel obligated to post something that is actually useful  Let s start with the shortest  but very strict way  var str    quot true quot   var mybool   JSON parse str    And end with a proper  more tolerant way  var parseBool   function str            console log typeof str          strict  JSON parse str           if str    null          return false           if  typeof str      boolean                 return  str     true                   if typeof str      string                 if str     quot  quot               return false                       str   str replace    s   s   g               if str toLowerCase       true     str toLowerCase       yes               return true                   str   str replace    g                str   str replace    s  - s  g   -                      var isNum   string match    0-9         null         var isNum      d    test str       if  isNaN str           return  parseFloat str     0                return false     Testing  var array 1   new Array true  1   quot 1 quot  -1   quot -1 quot    quot  - 1 quot    quot true quot    quot TrUe quot    quot   true   quot    quot   TrUe quot   1 0   quot 1 5 quot    quot 1 5 quot   1 5  5  -3  -0 1  0 1   quot  - 0 1 quot   Infinity   quot Infinity quot   -Infinity   quot -Infinity quot   quot  - Infinity quot    quot  yEs quot     var array 2   new Array null   quot  quot   false   quot false quot    quot    false    quot    quot  f alse quot    quot FaLsE quot   0   quot 00 quot    quot 1 0 quot   0 0   quot 0 0 quot    quot 0 0 quot    quot 100a quot    quot 1 00 quot    quot  0  quot   0 0   quot 0 0 quot   -0 0   quot -0 0 quot    quot  -1a  quot    quot abc quot      for var i  0  i  lt  array 1 length   i   console log  quot array 1  quot  i  quot     quot  array 1 i   quot     quot    parseBool array 1 i       for var i  0  i  lt  array 2 length   i   console log  quot array 2  quot  i  quot     quot  array 2 i   quot     quot    parseBool array 2 i       for var i  0  i  lt  array 1 length   i   console log parseBool array 1 i      for var i  0  i  lt  array 2 length   i   console log parseBool array 2 i

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I m using this one  String prototype maybeBool   function         if     yes    true    1    on   indexOf  this toLowerCase         -1   return true      if     no    false    0    off   indexOf  this toLowerCase         -1   return false       return this       on  maybeBool      returns true   off  maybeBool      returns false   I like js  maybeBool      returns  I like js

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I think it can be done in 1 liner with a use arrow function const convertStringToBoolean    value    gt  value   String value  toLowerCase        true    false   You guys can run and test various cases with following code snippet  x000D   x000D  const convertStringToBoolean    value    gt  value   String value  toLowerCase        true    false   console log convertStringToBoolean  a     console log convertStringToBoolean null    console log convertStringToBoolean undefined    console log convertStringToBoolean  undefined     console log convertStringToBoolean true    console log convertStringToBoolean false    console log convertStringToBoolean 0    console log convertStringToBoolean 1       only case which will not work x000D   x000D   x000D

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var myBool   Boolean parse  true      or  var myBool   Boolean  true      or   var myBool      true     Not sure if the first one is IE specific

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var trueVals     y    t    yes    true    gimme    var isTrueSet    trueVals indexOf myValue   gt  -1    true   false    or even just  var trueVals     y    t    yes    true    gimme    var isTrueSet    trueVals indexOf myValue   gt  -1     Similar to some of the switch statements but more compact  The value returned will only be true if the string is one of the trueVals strings  Everything else is false  Of course  you might want to normalise the input string to make it lower case and trim any spaces

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var falsy        f   alse   no  0    i  Boolean parse   function val         return  falsy test val   amp  amp    val       This returns false for every falsy value and true for every truthy value except for  false    f    no    n   and  0   case-insensitive       False Boolean parse false   Boolean parse  false    Boolean parse  False    Boolean parse  FALSE    Boolean parse  f    Boolean parse  F    Boolean parse  no    Boolean parse  No    Boolean parse  NO    Boolean parse  n    Boolean parse  N    Boolean parse  0    Boolean parse      Boolean parse 0   Boolean parse null   Boolean parse undefined   Boolean parse NaN   Boolean parse       True Boolean parse true   Boolean parse  true    Boolean parse  True    Boolean parse  t    Boolean parse  yes    Boolean parse  YES    Boolean parse  y    Boolean parse  1    Boolean parse  foo    Boolean parse      Boolean parse 1   Boolean parse -1   Boolean parse new Date

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You need to separate  in your thinking  the value of your selections and the representation of that value    Pick a point in the JavaScript logic where they need to transition from string sentinels to native type and do a comparison there  preferably where it only gets done once for each value that needs to be converted  Remember to address what needs to happen if the string sentinel is not one the script knows  i e  do you default to true or to false    In other words  yes  you need to depend on the string s value   -

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If there s some other code that s converting the boolean value to a string  you need to know exactly how that code stores true false values  Either that or you need to have access to a function that reverses that conversion   There are infinitely many ways to represent boolean values in strings   true    Y    1   etc    So you shouldn t rely on some general-purpose string-to-boolean converter  like Boolean myValue   You need to use a routine that reverses the original boolean-to-string conversion  whatever that is   If you know that it converts true booleans to  true  strings  then your sample code is fine  Except that you should use     instead of     so there s no automatic type conversion

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To Get Boolean values from string or number Here is good solution   var boolValue   Boolean Number  0      var boolValue   Boolean Number  1       First will return false and second will return true

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another solution   jsFiddle  var toBoolean   function value        var strValue   String value  toLowerCase        strValue      isNaN strValue   amp  amp  strValue      0    amp  amp          strValue         amp  amp          strValue      null   amp  amp          strValue      undefined      1    strValue      return strValue      true     strValue      1    true   false      test cases run in node   gt  toBoolean true  true  gt  toBoolean false  false  gt  toBoolean undefined  false  gt  toBoolean null  false  gt  toBoolean  true   true  gt  toBoolean  True   true  gt  toBoolean  False   false  gt  toBoolean  false   false  gt  toBoolean  0   false  gt  toBoolean  1   true  gt  toBoolean  100   true  gt

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In HTML the values of attributes eventually become strings  To mitigate that in undesired situations you can have a function to conditionally parse them into values they represent in the JavaScript or any other programming langauge of interest   Following is an explanation to do it for reviving boolean type from the string type  but it can be further expanded into other data types too  like numbers  arrays or objects   In addition to that JSON parse has a revive parameter which is a function  It also can be used to achieve the same   Let s call a string looking like a boolean   true   a boolean string likewise we can call a string like a number   1   a number string  Then we can determine if a string is a boolean string   const isBooleanString    string    gt    true    false   some item   gt  item     string     After that we need to parse the boolean string as JSON by JSON parse method   JSON parse aBooleanString     However  any string that is not a boolean string  number string  or any stringified object or array  any invalid JSON  will cause the JSON parse method to throw a SyntaxError   So  you will need to know with what to call it  i e  if it is a boolean string  You can achieve this by writing a function that makes the above defiend boolean string check and call JSON parse   function parse string     return isBooleanString string    JSON parse string        string      One can further generalize the isBooleanString utility to have a more broader perspective on what qualifies as a boolean string by further parametrizing it to accept an optional array of accepted boolean strings   const isBooleanString    string  spec     true    false    True    False      gt  spec some item   gt  item     string

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The following would be enough  String prototype boolean   function         return  true     this        true  boolean      returns true  false  boolean      returns false

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One Liner  We just need to account for the  false  string since any other string  including  true   is already true   function b v   return v    false    false     v      Test  b true       true b  true      true b false      false b  false     false     A more exaustive version  function bool v   return v    false     v    null     v    NaN     v    undefined     v    0    false     v      Test  bool true           true bool  true          true bool 1              true bool  1             true bool  hello         true  bool false          false bool  false         false bool 0              false bool  0             false bool null           false bool  null          false bool NaN            false bool  NaN           false bool undefined      false bool  undefined     false bool                false  bool                true bool                true bool alert          true bool window         true

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You can use regular expressions         Converts a string to a bool        This conversion will         - match  true    on   or  1  as true      - ignore all white-space padding     - ignore capitalization  case            tRue     ON   and  1     will all evaluate as true         function strToBool s           will match one and only one of the string  true   1   or  on  rerardless        of capitalization and regardless off surrounding white-space             regex    s  true 1 on  s   i      return regex test s       If you like extending the String class you can do   String prototype bool   function         return strToBool this       alert  true  bool       For those  see the comments  that would like to extend the String object to get this but are worried about enumerability and are worried about clashing with other code that extends the String object   Object defineProperty String prototype   com example bool         get   function             return     true 1   i  test this             alert  true  com example bool      Won t work in older browsers of course and Firefox shows false while Opera  Chrome  Safari and IE show true  Bug 720760

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MyLib Convert bool   function param             var res   String param  toLowerCase             return    Boolean res     res      false     res      0

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Simple solution i have been using it for a while  function asBoolean value         return     value       true            asBoolean true     gt  true    asBoolean false     gt  false    asBoolean  true      gt  true    asBoolean  false      gt  false

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I think it can be done in 1 liner with a use arrow function const convertStringToBoolean    value    gt  value   String value  toLowerCase        true    false   You guys can run and test various cases with following code snippet  x000D   x000D  const convertStringToBoolean    value    gt  value   String value  toLowerCase        true    false   console log convertStringToBoolean  a     console log convertStringToBoolean null    console log convertStringToBoolean undefined    console log convertStringToBoolean  undefined     console log convertStringToBoolean true    console log convertStringToBoolean false    console log convertStringToBoolean 0    console log convertStringToBoolean 1       only case which will not work x000D   x000D   x000D

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i wrote a helper function that handles your cases  and some more   Feel free to alter it to your specific needs          example     lt code gt     var pageRequestParams     enableFeatureX    true       toBool pageRequestParams enableFeatureX       returns true       toBool pageRequestParams enableFeatureY  true  options enableFeatureY      lt  code gt      param    value     param  Boolean  mapEmptyStringToTrue false      param  Boolean  defaultVal false  this is returned if value is undefined         returns  Boolean      example     lt code gt     toBool   enableFeatureX               enableFeatureX               false    toBool   enableFeatureX               enableFeatureX  true         true    toBool                                enableFeatureX  true         false    toBool   enableFeatureX   0           enableFeatureX               false    toBool   enableFeatureX    0          enableFeatureX               false    toBool   enableFeatureX    0          enableFeatureX               false    toBool   enableFeatureX    false      enableFeatureX               false    toBool   enableFeatureX    falsE      enableFeatureX               false    toBool   enableFeatureX    no         enableFeatureX               false       toBool   enableFeatureX   1           enableFeatureX               true    toBool   enableFeatureX    -2         enableFeatureX               true    toBool   enableFeatureX    true       enableFeatureX               true    toBool   enableFeatureX    false      enableFeatureX               true    toBool   enableFeatureX    john doe   enableFeatureX               true     lt  code gt         var toBool   function  value  mapEmptyStringToTrue  defaultVal        if  value     undefined   return Boolean defaultVal         mapEmptyStringToTrue   mapEmptyStringToTrue     undefined   mapEmptyStringToTrue   false     default to false     var strFalseValues     0    false    no   concat  mapEmptyStringToTrue                   if  typeof value      string             return  strFalseValues indexOf value toLowerCase   trim        -1                value is likely null  boolean  or number     return Boolean value

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why don t you try something like this  Boolean JSON parse  yourString toString    toLowerCase        It will return an error when some other text is given rather than true or false regardless of the case and it will capture the numbers also as      0- gt  false    any other number - gt  true

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i wrote a helper function that handles your cases  and some more   Feel free to alter it to your specific needs          example     lt code gt     var pageRequestParams     enableFeatureX    true       toBool pageRequestParams enableFeatureX       returns true       toBool pageRequestParams enableFeatureY  true  options enableFeatureY      lt  code gt      param    value     param  Boolean  mapEmptyStringToTrue false      param  Boolean  defaultVal false  this is returned if value is undefined         returns  Boolean      example     lt code gt     toBool   enableFeatureX               enableFeatureX               false    toBool   enableFeatureX               enableFeatureX  true         true    toBool                                enableFeatureX  true         false    toBool   enableFeatureX   0           enableFeatureX               false    toBool   enableFeatureX    0          enableFeatureX               false    toBool   enableFeatureX    0          enableFeatureX               false    toBool   enableFeatureX    false      enableFeatureX               false    toBool   enableFeatureX    falsE      enableFeatureX               false    toBool   enableFeatureX    no         enableFeatureX               false       toBool   enableFeatureX   1           enableFeatureX               true    toBool   enableFeatureX    -2         enableFeatureX               true    toBool   enableFeatureX    true       enableFeatureX               true    toBool   enableFeatureX    false      enableFeatureX               true    toBool   enableFeatureX    john doe   enableFeatureX               true     lt  code gt         var toBool   function  value  mapEmptyStringToTrue  defaultVal        if  value     undefined   return Boolean defaultVal         mapEmptyStringToTrue   mapEmptyStringToTrue     undefined   mapEmptyStringToTrue   false     default to false     var strFalseValues     0    false    no   concat  mapEmptyStringToTrue                   if  typeof value      string             return  strFalseValues indexOf value toLowerCase   trim        -1                value is likely null  boolean  or number     return Boolean value

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A shorter way to write this  could be var isTrueSet    myValue      true     true   false  Presuming only  true  is true and other values are false

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String true  toLowerCase       true      true String  true   toLowerCase       true      true String  True   toLowerCase       true      true String  TRUE   toLowerCase       true      true  String false  toLowerCase       true      false   If you are not sure of the input  the above works for boolean and as well any string

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function returnBoolean str        str str toString   toLowerCase         if str   true     str   1     str   yes     str   y     str   on     str                return true             else if str   false     str   0     str   no     str   n     str   off     str   -            return false        else          return undefined

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Hands down the easiest way  assuming you string will be  true  or  false   is   var z    true   var y    false   var b    z      true       will evaluate to true var c    y      true       will evaluate to false   Always use the     operator instead of the    operator for these types of conversions

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This function can handle string as well as Boolean true false   function stringToBoolean val       var a              true  true           false  false            return a val       Demonstration below      x000D   x000D  function stringToBoolean val    x000D    var a     x000D       true   true  x000D       false   false x000D       x000D    return a val   x000D    x000D   x000D  console log stringToBoolean  true     x000D   x000D  console log typeof stringToBoolean  true      x000D   x000D  console log stringToBoolean  false     x000D   x000D  console log typeof stringToBoolean  false      x000D   x000D  console log stringToBoolean true    x000D   x000D  console log typeof stringToBoolean true     x000D   x000D  console log stringToBoolean false    x000D   x000D  console log typeof stringToBoolean false     x000D   x000D  console log                                                   x000D     what if value was undefined   x000D  console log  undefined result       stringToBoolean undefined    x000D  console log  type of undefined result       typeof stringToBoolean undefined     x000D  console log                                                   x000D     what if value was an unrelated string  x000D  console log  unrelated string result       stringToBoolean  hello world     x000D  console log  type of unrelated string result       typeof stringToBoolean undefined     x000D   x000D   x000D

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The following would be enough  String prototype boolean   function         return  true     this        true  boolean      returns true  false  boolean      returns false

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You can use regular expressions         Converts a string to a bool        This conversion will         - match  true    on   or  1  as true      - ignore all white-space padding     - ignore capitalization  case            tRue     ON   and  1     will all evaluate as true         function strToBool s           will match one and only one of the string  true   1   or  on  rerardless        of capitalization and regardless off surrounding white-space             regex    s  true 1 on  s   i      return regex test s       If you like extending the String class you can do   String prototype bool   function         return strToBool this       alert  true  bool       For those  see the comments  that would like to extend the String object to get this but are worried about enumerability and are worried about clashing with other code that extends the String object   Object defineProperty String prototype   com example bool         get   function             return     true 1   i  test this             alert  true  com example bool      Won t work in older browsers of course and Firefox shows false while Opera  Chrome  Safari and IE show true  Bug 720760

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I ve found that using  1  and an empty value    for boolean values works far more predictably than  true  or  false  string values    specifically with html forms since uninitialized empty values in Dom elements will consistently evaluate to false whereas any value within them evaluates to true    For instance    lt input type  button  onclick  this value   tog this value      gt    lt script type  text javascript  gt       function tog off            if off                alert  true  toggle to false                return               else               alert  false  toggle to true                return  1                       lt  script gt    Just seemed like an easier road  so far it s been very consistent easy    perhaps someone can determine a way to break this

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Lots of fancy answers here  Really surprised no one has posted this solution   var booleanVal   toCast  gt        This resolves to true in most cases other than bool false  number zero and empty string  obviously   You can easily look for other falsey string values after the fact e g    var booleanVal   toCast  gt      amp  amp  toCast     false   amp  amp  toCast     0

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I hope this is a most comprehensive use case  function parseBoolean token      if  typeof token      string         switch  token toLowerCase            case  on         case  yes         case  ok         case  ja         case                case              case             token   true          break        default          token   false              let ret   false    try       ret   Boolean JSON parse token        catch  e           do nothing or make a notification       return ret

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if you are sure the input is anything only within  true  and  false  why not    x000D   x000D  let x    true      let x   false   let y   x    true  true false  console log typeof y   y   x000D   x000D   x000D

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Remember to match case   var isTrueSet    myValue toLowerCase        true      Also  if it s a form element checkbox  you can also detect if the checkbox is checked   var isTrueSet   document myForm IS TRUE checked    Assuming that if it is checked  it is  set  equal to true  This evaluates as true false

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guinaps  Any string which isn t the empty string will evaluate to true by using them   How about using the String match   method  var str  true   var boolStr Boolean str match   true  i       this alone won t get the 1 0 or the yes no  but it will catch the TRUE true  as well  it will return false for any string that happens to have   true  as a substring   EDIT  Below is a function to handle true false  1 0  yes no  case-insensitive    function stringToBool str        var bool      if  str match    true 1 yes   i      null            bool   true        else if  str match    false 0 no    i      null            bool   false        else           bool   null          if  console  console log       str      is not a boolean value              return bool     stringToBool  1       true stringToBool  No       false stringToBool  falsey       null   falsey  is not a boolean value   stringToBool         false

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You can use Function to return a Boolean value from string  true  or  false    x000D   x000D  const TRUE OR FALSE   str   gt  new Function  return   str       x000D   x000D  const  TRUE  FALSE      true    false    x000D   x000D  const  T  F     TRUE OR FALSE TRUE   TRUE OR FALSE FALSE    x000D   x000D  console log T  typeof T       true    boolean   x000D   x000D  console log F  typeof F       false    boolean   x000D   x000D   x000D

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The strongest way is the following because it also handle undefined case         true   true   false   false   myValue           true   true   false   false   undefined       gt  undefined        true   true   false   false    true        gt  true        true   true   false   false    false       gt  false

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another solution   jsFiddle  var toBoolean   function value        var strValue   String value  toLowerCase        strValue      isNaN strValue   amp  amp  strValue      0    amp  amp          strValue         amp  amp          strValue      null   amp  amp          strValue      undefined      1    strValue      return strValue      true     strValue      1    true   false      test cases run in node   gt  toBoolean true  true  gt  toBoolean false  false  gt  toBoolean undefined  false  gt  toBoolean null  false  gt  toBoolean  true   true  gt  toBoolean  True   true  gt  toBoolean  False   false  gt  toBoolean  false   false  gt  toBoolean  0   false  gt  toBoolean  1   true  gt  toBoolean  100   true  gt

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you can use JSON parse as follows   x000D   x000D      var trueOrFalse  True   result  JSON parse trueOrFalse toLowerCase     if result  true    alert  this is true    else    alert  this is false    x000D   x000D   x000D   in this case  toLowerCase is important

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Do   var isTrueSet    myValue     true      You could make it stricter by using the identity operator        which doesn t make any implicit type conversions when the compared variables have different types  instead of the equality operator        var isTrueSet    myValue      true        Don t   You should probably be cautious about using these two methods for your specific needs   var myBool   Boolean  false           true  var myBool      false          true   Any string which isn t the empty string will evaluate to true by using them  Although they re the cleanest methods I can think of concerning to boolean conversion  I think they re not what you re looking for

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I use this simple approach  using  myVarToTest     var trueValuesRange     1   1   true   true    myVarToTest    trueValuesRange indexOf myVarToTest   gt   0

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function convertBoolean value   boolean       if  typeof value     string             value   value toLowerCase              switch  value            case true          case  quot true quot           case  quot evet quot      Locale         case  quot t quot           case  quot e quot      Locale         case  quot 1 quot           case  quot on quot           case  quot yes quot           case 1              return true          case false          case  quot false quot           case  quot hayir quot      Locale         case  quot f quot           case  quot h quot      Locale         case  quot 0 quot           case  quot off quot           case  quot no quot           case 0              return false          default              return null

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Use an if statment   x000D   x000D  function parseBool str      if  str toLowerCase       true         var val   true      else if  str toLowerCase       false         var val   false      else         If it is not true of false it returns undefined        var val   undefined        return val    console log parseBool      typeof parseBool       console log parseBool  TrUe    typeof parseBool  TrUe     console log parseBool  false    typeof parseBool  false     x000D   x000D   x000D

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Convert String to Boolean  var vIn    true   var vOut   vIn toLowerCase     true  1 0    Convert String to Number  var vIn   0  var vOut   parseInt vIn 10  base

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var trueVals     y    t    yes    true    gimme    var isTrueSet    trueVals indexOf myValue   gt  -1    true   false    or even just  var trueVals     y    t    yes    true    gimme    var isTrueSet    trueVals indexOf myValue   gt  -1     Similar to some of the switch statements but more compact  The value returned will only be true if the string is one of the trueVals strings  Everything else is false  Of course  you might want to normalise the input string to make it lower case and trim any spaces

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You even do not need to convert the string to boolean  just use the following  var yourstring   yourstringValue     1   true   false

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Boolean parse   function  str      switch  str toLowerCase           case  true         return true      case  false         return false      default        throw new Error   Boolean parse  Cannot convert string to boolean

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Your solution is fine   Using     would just be silly in this case  as the field s value will always be a String

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The simplest way which I always use   let value    true   let output   value      true

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Remember to match case   var isTrueSet    myValue toLowerCase        true      Also  if it s a form element checkbox  you can also detect if the checkbox is checked   var isTrueSet   document myForm IS TRUE checked    Assuming that if it is checked  it is  set  equal to true  This evaluates as true false

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You don t even need to use a variable  if you know that  true  will always be lowercase you can use this which will return true or false   eval yourBooleanString     true

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Holy god some of these answers are just wild   I love JS and its infinite number of ways to skin a bool   My preference  which I was shocked not to see already  is   testVar   testVar toString   match    true  1-9  0-9    0-9   1-9   yes   i    true   false

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You can use regular expressions         Converts a string to a bool        This conversion will         - match  true    on   or  1  as true      - ignore all white-space padding     - ignore capitalization  case            tRue     ON   and  1     will all evaluate as true         function strToBool s           will match one and only one of the string  true   1   or  on  rerardless        of capitalization and regardless off surrounding white-space             regex    s  true 1 on  s   i      return regex test s       If you like extending the String class you can do   String prototype bool   function         return strToBool this       alert  true  bool       For those  see the comments  that would like to extend the String object to get this but are worried about enumerability and are worried about clashing with other code that extends the String object   Object defineProperty String prototype   com example bool         get   function             return     true 1   i  test this             alert  true  com example bool      Won t work in older browsers of course and Firefox shows false while Opera  Chrome  Safari and IE show true  Bug 720760

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Simplest solution  with ES6  use the logical NOT twice        to get the string converted Just paste this expression    const stringToBoolean    string    gt  string      false    false     string  And pass your string to it  stringToBoolean                        false stringToBoolean  false                 false stringToBoolean  true                  true stringToBoolean  hello my friend       true   Bonus   const betterStringToBoolean    string    gt     string      false     string      undefined     string      null     string      0      false     string  You can include other strings at will to easily extend the usage of this expression     betterStringToBoolean  undefined          false betterStringToBoolean  null               false betterStringToBoolean  0                  false betterStringToBoolean  false              false betterStringToBoolean                     false betterStringToBoolean  true               true betterStringToBoolean  anything else      true

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if  String a      true        true block   else       false block

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If there s some other code that s converting the boolean value to a string  you need to know exactly how that code stores true false values  Either that or you need to have access to a function that reverses that conversion   There are infinitely many ways to represent boolean values in strings   true    Y    1   etc    So you shouldn t rely on some general-purpose string-to-boolean converter  like Boolean myValue   You need to use a routine that reverses the original boolean-to-string conversion  whatever that is   If you know that it converts true booleans to  true  strings  then your sample code is fine  Except that you should use     instead of     so there s no automatic type conversion

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The Boolean object doesn t have a  parse  method  Boolean  false   returns true  so that won t work     false  also returns true  so that won t work also   If you want string  true  to return boolean true and string  false  to return boolean false  then the simplest solution is to use eval    eval  true   returns true and eval  false   returns false  Keep in mind the performance implications when using eval   though

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Here is my 1 liner submission  I needed to evaluate a string and output  true if  true   false if  false  and a number if anything like  -12 35673    val    false    val     false  i test val    false       true  i test val    true   val 1   val 1   val

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I thought that  Steven  s answer was the best one  and took care of a lot more cases than if the incoming value was just a string  I wanted to extend it a bit and offer the following   function isTrue value       if  typeof value       string            value   value trim   toLowerCase              switch value           case true          case  true           case 1          case  1           case  on           case  yes               return true          default               return false            It s not necessary to cover all the false cases if you already know all of the true cases you d have to account for  You can pass anything into this method that could pass for a true value  or add others  it s pretty straightforward   and everything else would be considered false

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Your solution is fine   Using     would just be silly in this case  as the field s value will always be a String

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I do this  which will handle 1 TRUE yes YES true  0 FALSE no NO false   BOOL false if  STRING    BOOL JSON parse STRING toLowerCase   replace  no   false   replace  yes   true       Replace STRING with the name of your string variable   If it s not null  a numerical value or one of these strings   true    TRUE    false    FALSE    yes    YES    no    NO  It will throw an error  intentionally

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works perfectly and very simple    var boolean    false   boolean    boolean      true       boolean   JSON parse boolean     or this way      to test it     x000D   x000D  var boolean    false   x000D  boolean    boolean      true    x000D   x000D    boolean   JSON parse boolean     or this way    x000D   x000D  if boolean    true   x000D      alert  boolean     boolean   x000D   else  x000D      alert  boolean     boolean   x000D    x000D   x000D   x000D

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Many of the existing answers use an approach that is semantically similar to this  but I think there is value in mentioning that the following  one liner  is often sufficient  For example  in addition to the OP s case  strings in a form  one often wants to read environment variables from process env in NodeJS  whose values  to the best of my knowledge  are always strings  in order to enable or disable certain behaviors  and it is common for these to have the form SOME ENV VAR 1   const toBooleanSimple    input    gt       t    y    1   some truePrefix   gt  truePrefix     input 0  toLowerCase       A slightly more robust and expressive implementation might look like this          Converts strings to booleans in a manner that is less surprising    to the non-JS world  e g  returns true for  1    yes    True   etc     and false for  0    No    false   etc       param input     returns  boolean      function toBoolean input      if  typeof input      string         return Boolean input         const s   input toLowerCase      return   t    y    1   some prefix   gt  s startsWith prefix        A  jest  unit test for this might look like this   describe  toBoolean   function       const groups          inputs    y    Yes    true    1   true  1       expectedOutput  true            inputs    n    No    false    0   false  0       expectedOutput  false        for  let group of groups        for  let input of group inputs          it  should return   group expectedOutput  for   JSON stringify input     function             expect toBoolean input   toEqual group expectedOutput

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In nodejs by using node-boolify it is possible  Boolean Conversion Results  Boolify true     true Boolify  true      true Boolify  TRUE      null Boolify 1     true Boolify 2     null Boolify false     false Boolify  false      false Boolify  FALSE      null Boolify 0     false Boolify null     null Boolify undefined     null Boolify      null Boolify        null

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I use an own method which includes a check if the object exists first and a more intuitive conversion to boolean   function str2bool strvalue     return  strvalue  amp  amp  typeof strvalue     string      strvalue toLowerCase       true     strvalue     1      strvalue    true       The results are   var test     false var test2   null     false var test3    undefined      false var test4    true      true var test5    false      false var test6   true     true var test7   false     false var test8   1     true var test9   0     false var test10    1      true var test11    0      false   Fiddle  http   jsfiddle net av5xcj6s

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Wood-eye be careful  After seeing the consequences after applying the top answer with 500  upvotes  I feel obligated to post something that is actually useful  Let s start with the shortest  but very strict way  var str    quot true quot   var mybool   JSON parse str    And end with a proper  more tolerant way  var parseBool   function str            console log typeof str          strict  JSON parse str           if str    null          return false           if  typeof str      boolean                 return  str     true                   if typeof str      string                 if str     quot  quot               return false                       str   str replace    s   s   g               if str toLowerCase       true     str toLowerCase       yes               return true                   str   str replace    g                str   str replace    s  - s  g   -                      var isNum   string match    0-9         null         var isNum      d    test str       if  isNaN str           return  parseFloat str     0                return false     Testing  var array 1   new Array true  1   quot 1 quot  -1   quot -1 quot    quot  - 1 quot    quot true quot    quot TrUe quot    quot   true   quot    quot   TrUe quot   1 0   quot 1 5 quot    quot 1 5 quot   1 5  5  -3  -0 1  0 1   quot  - 0 1 quot   Infinity   quot Infinity quot   -Infinity   quot -Infinity quot   quot  - Infinity quot    quot  yEs quot     var array 2   new Array null   quot  quot   false   quot false quot    quot    false    quot    quot  f alse quot    quot FaLsE quot   0   quot 00 quot    quot 1 0 quot   0 0   quot 0 0 quot    quot 0 0 quot    quot 100a quot    quot 1 00 quot    quot  0  quot   0 0   quot 0 0 quot   -0 0   quot -0 0 quot    quot  -1a  quot    quot abc quot      for var i  0  i  lt  array 1 length   i   console log  quot array 1  quot  i  quot     quot  array 1 i   quot     quot    parseBool array 1 i       for var i  0  i  lt  array 2 length   i   console log  quot array 2  quot  i  quot     quot  array 2 i   quot     quot    parseBool array 2 i       for var i  0  i  lt  array 1 length   i   console log parseBool array 1 i      for var i  0  i  lt  array 2 length   i   console log parseBool array 2 i

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you can use JSON parse as follows   x000D   x000D      var trueOrFalse  True   result  JSON parse trueOrFalse toLowerCase     if result  true    alert  this is true    else    alert  this is false    x000D   x000D   x000D   in this case  toLowerCase is important

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The expression you re looking for simply is    true  i test myValue    as in  var isTrueSet     true  i test myValue     This tests myValue against a regular expression   case-insensitive  and doesn t modify the prototype   Examples     true  i test  true       true   true  i test  TRUE       true   true  i test  tRuE       true   true  i test   tRuE       false  notice the space at the beginning    true  i test  untrue       false  some other solutions here will incorrectly return true   true  i test  false      returns false   true  i test  xyz        returns false

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Remember to match case   var isTrueSet    myValue toLowerCase        true      Also  if it s a form element checkbox  you can also detect if the checkbox is checked   var isTrueSet   document myForm IS TRUE checked    Assuming that if it is checked  it is  set  equal to true  This evaluates as true false

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Warning  This highly upvoted legacy answer is technically correct but only covers a very specific scenario  when your string value is EXACTLY  true  or  false    An invalid json string passed into these functions below WILL throw an exception     Original answer   How about   JSON parse  True  toLowerCase       or with jQuery    parseJSON  TRUE  toLowerCase

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function convertBoolean value   boolean       if  typeof value     string             value   value toLowerCase              switch  value            case true          case  quot true quot           case  quot evet quot      Locale         case  quot t quot           case  quot e quot      Locale         case  quot 1 quot           case  quot on quot           case  quot yes quot           case 1              return true          case false          case  quot false quot           case  quot hayir quot      Locale         case  quot f quot           case  quot h quot      Locale         case  quot 0 quot           case  quot off quot           case  quot no quot           case 0              return false          default              return null

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In nodejs by using node-boolify it is possible  Boolean Conversion Results  Boolify true     true Boolify  true      true Boolify  TRUE      null Boolify 1     true Boolify 2     null Boolify false     false Boolify  false      false Boolify  FALSE      null Boolify 0     false Boolify null     null Boolify undefined     null Boolify      null Boolify        null

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var myBool   Boolean parse  true      or  var myBool   Boolean  true      or   var myBool      true     Not sure if the first one is IE specific

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Do   var isTrueSet    myValue     true      You could make it stricter by using the identity operator        which doesn t make any implicit type conversions when the compared variables have different types  instead of the equality operator        var isTrueSet    myValue      true        Don t   You should probably be cautious about using these two methods for your specific needs   var myBool   Boolean  false           true  var myBool      false          true   Any string which isn t the empty string will evaluate to true by using them  Although they re the cleanest methods I can think of concerning to boolean conversion  I think they re not what you re looking for

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I m using this one when I get value from URL Form or other source   It is pretty universal one line piece of code   Maybe not the best for performance  if you need to run it millions times let me know  we can check how to optimize it  otherwise is pretty good and customizable   boolResult       false    0        undefined   indexOf String myVar  toLowerCase   trim      1     Result   myVar   true      true myVar    true       true myVar    TRUE       true myVar    1       true myVar    any other value not related to false       true  myVar   false     false myVar    false       false myVar    FALSE       false myVar    0       false

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Take it easy using this lib   https   github com rohmanhm force-boolean  you just need to write a single line  const ForceBoolean   require  force-boolean    const YOUR VAR    false  console log ForceBoolean YOUR VAR      it s return boolean false   It s also support for following   return false if value is number 0  return false if value is string  0   return false if value is string  false   return false if value is boolean false  return true if value is number 1  return true if value is string  1   return true if value is string  true   return true if value is boolean true

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I m using this one when I get value from URL Form or other source   It is pretty universal one line piece of code   Maybe not the best for performance  if you need to run it millions times let me know  we can check how to optimize it  otherwise is pretty good and customizable   boolResult       false    0        undefined   indexOf String myVar  toLowerCase   trim      1     Result   myVar   true      true myVar    true       true myVar    TRUE       true myVar    1       true myVar    any other value not related to false       true  myVar   false     false myVar    false       false myVar    FALSE       false myVar    0       false

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Simple one line operation if you need Boolean false and true from the string values  storeBooleanHere   stringVariable   quot true quot  true false    storeBooleanHere - This variable will hold the boolean value stringVariable - Variable that has boolean stored as string

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You don t even need to use a variable  if you know that  true  will always be lowercase you can use this which will return true or false   eval yourBooleanString     true

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Use an if statment   x000D   x000D  function parseBool str      if  str toLowerCase       true         var val   true      else if  str toLowerCase       false         var val   false      else         If it is not true of false it returns undefined        var val   undefined        return val    console log parseBool      typeof parseBool       console log parseBool  TrUe    typeof parseBool  TrUe     console log parseBool  false    typeof parseBool  false     x000D   x000D   x000D

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One Liner  We just need to account for the  false  string since any other string  including  true   is already true   function b v   return v    false    false     v      Test  b true       true b  true      true b false      false b  false     false     A more exaustive version  function bool v   return v    false     v    null     v    NaN     v    undefined     v    0    false     v      Test  bool true           true bool  true          true bool 1              true bool  1             true bool  hello         true  bool false          false bool  false         false bool 0              false bool  0             false bool null           false bool  null          false bool NaN            false bool  NaN           false bool undefined      false bool  undefined     false bool                false  bool                true bool                true bool alert          true bool window         true

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Hands down the easiest way  assuming you string will be  true  or  false   is   var z    true   var y    false   var b    z      true       will evaluate to true var c    y      true       will evaluate to false   Always use the     operator instead of the    operator for these types of conversions

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The simplest way to convert a string to a boolean is the following  Boolean  lt stringVariable gt

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Boolean parse   does exist in some browser implementations   It s definitely not universal  so if that s something that you need than you shouldn t use this method   But in Chrome  for example  I m using v21  it works just fine and as one would expect

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The expression you re looking for simply is    true  i test myValue    as in  var isTrueSet     true  i test myValue     This tests myValue against a regular expression   case-insensitive  and doesn t modify the prototype   Examples     true  i test  true       true   true  i test  TRUE       true   true  i test  tRuE       true   true  i test   tRuE       false  notice the space at the beginning    true  i test  untrue       false  some other solutions here will incorrectly return true   true  i test  false      returns false   true  i test  xyz        returns false

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The most simple way is   a    True   a     a  amp  amp    1    true   1  true  indexOf a toLowerCase     gt  -1

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Just do a   var myBool   eval  yourString     Examples   alert  eval   true      true      TRUE alert  eval   true      false      FALSE alert  eval   1      true      TRUE alert  eval   1      false      FALSE alert  eval   false      true      FALSE  alert  eval   false      false      TRUE alert  eval   0      true      FALSE alert  eval   0      false      TRUE alert  eval         undefined      TRUE alert  eval       undefined      TRUE   This method handles the empty string and undefined string naturally as if you declare a variable without assigning it a value

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Convert String to Boolean  var vIn    true   var vOut   vIn toLowerCase     true  1 0    Convert String to Number  var vIn   0  var vOut   parseInt vIn 10  base

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I think this is much universal  if  String a  toLowerCase       quot true quot       It goes  String true      quot true quot        returns true String false      quot true quot       returns false String  quot true quot       quot true quot      returns true String  quot false quot       quot true quot     returns false

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Just do a   var myBool   eval  yourString     Examples   alert  eval   true      true      TRUE alert  eval   true      false      FALSE alert  eval   1      true      TRUE alert  eval   1      false      FALSE alert  eval   false      true      FALSE  alert  eval   false      false      TRUE alert  eval   0      true      FALSE alert  eval   0      false      TRUE alert  eval         undefined      TRUE alert  eval       undefined      TRUE   This method handles the empty string and undefined string naturally as if you declare a variable without assigning it a value

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A shorter way to write this  could be var isTrueSet    myValue      true     true   false  Presuming only  true  is true and other values are false

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const result  Boolean   strValue      true    true   false

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The  toBoolean  function returns false for null  undefined       false   It returns true for any other string   const toBoolean    bool    gt      if  bool      false   bool   false   return   bool    toBoolean  false      returns false

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Boolean parse   function  str      switch  str toLowerCase           case  true         return true      case  false         return false      default        throw new Error   Boolean parse  Cannot convert string to boolean

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Lots of fancy answers here  Really surprised no one has posted this solution   var booleanVal   toCast  gt        This resolves to true in most cases other than bool false  number zero and empty string  obviously   You can easily look for other falsey string values after the fact e g    var booleanVal   toCast  gt      amp  amp  toCast     false   amp  amp  toCast     0

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I m a little late  but I have a little snippet to do this  it essentially maintains all of JScripts truthey falsey filthy-ness but includes  false  as an acceptible value for false   I prefer this method to the ones mentioned because it doesn t rely on a 3rd party to parse the code  i e  eval JSON parse   which is overkill in my mind  it s short enough to not require a utility function and maintains other truthey falsey conventions   var value    false   var result    value     false      Boolean value       value    true     gt  result   true    value    false    gt  result   false    value   true      gt  result   true    value   false     gt  result   false    value   null      gt  result   false    value             gt  result   true    etc

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There are already so many answers available  But following can be useful in some scenarios      One can specify all values against which you consider truthy var TRUTHY VALUES    true   true   1    function getBoolean a        return TRUTHY VALUES some function t            return t     a              This can be useful where one examples with non-boolean values   getBoolean  aa       false getBoolean false     false getBoolean  false      false  getBoolean  true       true getBoolean true      true getBoolean 1      true

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I recommend you to create a function like the third option in the image and place it in a helper class as export  and reuse this function when you need

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Simplest solution  with ES6  use the logical NOT twice        to get the string converted Just paste this expression    const stringToBoolean    string    gt  string      false    false     string  And pass your string to it  stringToBoolean                        false stringToBoolean  false                 false stringToBoolean  true                  true stringToBoolean  hello my friend       true   Bonus   const betterStringToBoolean    string    gt     string      false     string      undefined     string      null     string      0      false     string  You can include other strings at will to easily extend the usage of this expression     betterStringToBoolean  undefined          false betterStringToBoolean  null               false betterStringToBoolean  0                  false betterStringToBoolean  false              false betterStringToBoolean                     false betterStringToBoolean  true               true betterStringToBoolean  anything else      true

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Take care  maybe in the future the code change and return boolean instead of one string at the moment   The solution would be     Currently var isTrue    true     In the future  Other developer change the code  var isTrue   true    The solution to both cases  isTrue  toString       true

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The  toBoolean  function returns false for null  undefined       false   It returns true for any other string   const toBoolean    bool    gt      if  bool      false   bool   false   return   bool    toBoolean  false      returns false

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Do   var isTrueSet    myValue     true      You could make it stricter by using the identity operator        which doesn t make any implicit type conversions when the compared variables have different types  instead of the equality operator        var isTrueSet    myValue      true        Don t   You should probably be cautious about using these two methods for your specific needs   var myBool   Boolean  false           true  var myBool      false          true   Any string which isn t the empty string will evaluate to true by using them  Although they re the cleanest methods I can think of concerning to boolean conversion  I think they re not what you re looking for

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My take on this question is that it aims to satisfy three objectives    Return true false for truthy and falsey values  but also return true false for multiple string values that would be truthy or falsey if they were Booleans instead of strings  Second  provide a resilient interface so that values other than those specified will not fail  but rather return a default value Third  do all this with as little code as possible    The problem with using JSON is that it fails by causing a Javascript error  This solution is not resilient  though it satisfies 1 and 3    JSON parse  FALSE      fails   This solution is not concise enough   if value      TRUE     value      yes            return true      I am working on solving this exact problem for Typecast js  And the best solution to all three objectives is this one   return   true  i test v     It works for many cases  does not fail when values like    are passed in  and is very concise  Also it returns false as the default value rather than undefined or throwing an Error  which is more useful in loosely-typed Javascript development  Bravo to the other answers that suggested it

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Convert something to boolean function toBoolean  o         if   null     o             let t   typeof o          if    quot undefined quot      typeof o                 if    quot string quot      t   return   o              o   o toLowerCase   trim                return  quot true quot      o     quot 1 quot      o                      return false     toBoolean false  -- gt  false toBoolean true  -- gt  true toBoolean  quot false quot   -- gt  false toBoolean  quot true quot   -- gt  true toBoolean  quot TRue quot   -- gt  true toBoolean  quot 1 quot   -- gt  true toBoolean  quot 0 quot   -- gt  false toBoolean 1  -- gt  true toBoolean 0  -- gt  false toBoolean 123 456  -- gt  true toBoolean 0 0  -- gt  false toBoolean  quot  quot   -- gt  false toBoolean null  -- gt  false toBoolean   -- gt  false

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function returnBoolean str        str str toString   toLowerCase         if str   true     str   1     str   yes     str   y     str   on     str                return true             else if str   false     str   0     str   no     str   n     str   off     str   -            return false        else          return undefined

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This has been taken from the accepted answer  but really it has a very weak point  and I am shocked how it got that count of upvotes  the problem with it that you have to consider the case of the string because this is case sensitive  var isTrueSet    myValue toLowerCase        true

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Your solution is fine   Using     would just be silly in this case  as the field s value will always be a String

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For TypeScript we can use the function   export function stringToBoolean s  string  valueDefault  boolean   false   boolean       switch s toLowerCase                  case  true           case  1           case  on           case  yes           case  y               return true           case  false           case  0           case  off           case  no           case  n               return false             return valueDefault

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I ve found that using  1  and an empty value    for boolean values works far more predictably than  true  or  false  string values    specifically with html forms since uninitialized empty values in Dom elements will consistently evaluate to false whereas any value within them evaluates to true    For instance    lt input type  button  onclick  this value   tog this value      gt    lt script type  text javascript  gt       function tog off            if off                alert  true  toggle to false                return               else               alert  false  toggle to true                return  1                       lt  script gt    Just seemed like an easier road  so far it s been very consistent easy    perhaps someone can determine a way to break this

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Here is simple function that will do the trick      function convertStringToBool str           return   str      True       str      true      true false          This will give the following result  convertStringToBool  false     returns false convertStringToBool  true      returns true convertStringToBool  False      returns false convertStringToBool  True      returns true

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A lot of the existing answers are similar  but most ignore the fact that the given argument could also be an object   Here is something I just whipped up   Utils parseBoolean   function val       if  typeof val      string     val instanceof String           return  true i test val         else if  typeof val      boolean     val instanceof Boolean           return new Boolean val  valueOf          else if  typeof val      number     val instanceof Number           return new Number val  valueOf       0            return false          and the unit test for it  Utils Tests   function        window console log  running unit tests         var booleanTests               true   true             false   false             True   true             False   false              false            true  true            false  false             gibberish   false            0  false            1  true              for  var i   0  i  lt  booleanTests length  i             var lhs   Utils parseBoolean booleanTests i  0            var rhs   booleanTests i  1           var result   lhs     rhs           if  result               console log  Utils parseBoolean   booleanTests i  0           booleanTests i  1    t    tpass              else               console log  Utils parseBoolean   booleanTests i  0           booleanTests i  1    t    tfail

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This function can handle string as well as Boolean true false   function stringToBoolean val       var a              true  true           false  false            return a val       Demonstration below      x000D   x000D  function stringToBoolean val    x000D    var a     x000D       true   true  x000D       false   false x000D       x000D    return a val   x000D    x000D   x000D  console log stringToBoolean  true     x000D   x000D  console log typeof stringToBoolean  true      x000D   x000D  console log stringToBoolean  false     x000D   x000D  console log typeof stringToBoolean  false      x000D   x000D  console log stringToBoolean true    x000D   x000D  console log typeof stringToBoolean true     x000D   x000D  console log stringToBoolean false    x000D   x000D  console log typeof stringToBoolean false     x000D   x000D  console log                                                   x000D     what if value was undefined   x000D  console log  undefined result       stringToBoolean undefined    x000D  console log  type of undefined result       typeof stringToBoolean undefined     x000D  console log                                                   x000D     what if value was an unrelated string  x000D  console log  unrelated string result       stringToBoolean  hello world     x000D  console log  type of unrelated string result       typeof stringToBoolean undefined     x000D   x000D   x000D

[javascript] How can I convert a string to boolean in JavaScript?

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