Check if a variable is a string in JavaScript

Question

How can I determine whether a variable is a string or something else in JavaScript

User · Answer

A simple solution would be   var x    hello   if x     x toString        it s a string   else     it isn t

User · Answer

I like to use this simple solution   var myString    test   if myString constructor     String           It s a string

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I recommend using the built-in functions from jQuery or lodash Underscore   They re simpler to use and easier to read   Either function will handle the case DRAX mentioned     that is  they both check if  A  the variable is a string literal or  B  it s an instance of the String object   In either case  these functions correctly identify the value as being a string   lodash   Underscore js  if   isString myVar        it s a string else      it s something else   jQuery  if   type myVar       string        it s a string else      it s something else   See lodash Documentation for   isString   for more details   See jQuery Documentation for   type   for more details

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function isString  obj      return  Object prototype toString call obj        object String         I saw that here   http   perfectionkills com instanceof-considered-harmful-or-how-to-write-a-robust-isarray

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This is what works for me   if  typeof myVar      string     myVar instanceof String     it s a string else    it s something else

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I think that  customcommander solution should suffice in 90  of your cases   typeof str      string    Should serve you right  simply since normally there s no reason to have new String  something   in your code    If you re interested in handling the String object as well  for example you expect some var from a 3rd party  then using lodash as  ClearCloud8 suggested seems like a clear  simple and elegant solution   I would however suggest to be cautious with libraries such as lodash due to their size  Instead of doing  import   from  lodash        isString myVar    Which brings the whole huge lodash object  I d suggest something like   import   isString as  isString   from  lodash       isString myVar    And with simple bundling you should be fine  I refer here to client code

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Taken from lodash   function isString val       return typeof val      string         val  amp  amp  typeof val      object    amp  amp  Object prototype toString call val        object String        console log isString  hello world         true console log isString new String  hello world         true

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Best way  var s    String   var a    1 2 3   var o    key   val      s constructor     String   amp  amp  console log  its a string     a constructor     Array   amp  amp  console log  its an array     o constructor     Object   amp  amp  console log  its an object     o constructor     Number    s constructor     Boolean   amp  amp  console log  this won  t run     Each of these has been constructed by its appropriate class function  like  quot new Object   quot  etc  Also  Duck-Typing   quot If it looks like a duck  walks like a duck  and smells like a duck - it must be an Array quot  Meaning  check its properties  Hope this helps  Edit  12 05 2016 Remember  you can always use combinations of approaches too  Here s an example of using an inline map of actions with typeof  var type      number   Math sqrt bind Math          typeof datum     Here s a more  real world  example of using inline-maps  function is datum        var isnt      null  true  undefined  true      true  false  false  0  false    datum        return  isnt    console log  is 0   is false   is undefined               gt  gt  true true false  This function would use   custom    quot type-casting quot  -- rather   quot type- -value-mapping quot  -- to figure out if a variable actually  quot exists quot   Now you can split that nasty hair between null  amp  0  Many times you don t even care about its type  Another way to circumvent typing is combining Duck-Type sets  this id    quot 998 quot       use a number or a string-equivalent function get id        if   id     id toString  return      if  id toString       this id toString    http  id     this id           if   id      this id          Both Number prototype and String prototype have a  toString   method  You just made sure that the string-equivalent of the number was the same  and then you made sure that you passed it into the http function as a Number  In other words  we didn t even care what its type was  Hope that gives you more to work with

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I m going to go a different route to the rest here  which try to tell if a variable is a specific  or a member of a specific set  of types  JS is built on ducktyping  if something quacks like a string  we can and should use it like a string   Is 7 a string  Then why does   d  test 7  work  Is  toString     gt   hello there    a string  Then why does   toString     gt   hello there         ngeneral kenobi   work  These aren t questions about should the above work  the point is they do   So I made a duckyString   function Below I test many cases not catered for by other answers  For each the code    sets a string-like variable runs an identical string operation on it and a real string to compare outputs  proving they can be treated like strings  converts the string-like to a real string to show you duckyString   to normalise inputs for code that expects real strings     text    hello there   out text replace  e g   E            hello there  replace  e g   E     out  Is string      duckyString text      t     duckyString text  true       n     text   new String  oh my    out text toUpperCase            oh my  toUpperCase     out  Is string      duckyString text      t     duckyString text  true       n     text   368  out  text     is a big number             368      is a big number     out  Is string      duckyString text      t     duckyString text  true       n     text      uD83D     uDE07    out text 1  charCodeAt 0             1  charCodeAt 0    out  Is string      duckyString text      t     duckyString text  true       n     function Text     this math   7     Text prototype    toString function     return this math   3          text   new Text    out String prototype match call text   0           text toString   match  0     out  Is string      duckyString text      t     duckyString text  true       n      This is in the same vein as   x as opposed to x   true and testing if something is array-like instead of necessitating an actual array  jQuery objects  are they arrays  No  Are they good enough  Yeah  you can run them through Array prototype functions just fine  It s this flexibility that gives JS its power  and testing for strings specifically makes your code less interoperable   The output of the above is   hEllo thErE hEllo thErE Is string  true  hello there   OH MY OH MY Is string  true  oh my   368 is a big number 368 is a big number Is string  true  368   56839 56839 Is string  true     0 0 Is string  true  10    So  it s all about why you want to know if something s a string  If  like me  you arrived here from google and wanted to see if something was string-like  here s an answer  It isn t even expensive unless you re working with really long or deeply nested char arrays  This is because it is all if statements  no function calls like  toString    Except if you re trying to see if a char array with objects that only have toString   s or multi-byte characters  in which case there s no other way to check except to make the string  and count characters the bytes make up  respectively  function duckyString string  normalise  unacceptable        var type   null      if   unacceptable          unacceptable           if  string  amp  amp   unacceptable chars  amp  amp  unacceptable to    null          unacceptable to   string toString    Array prototype toString       if  string    null                   tests if  string  just is a string     else if            unacceptable is  amp  amp           typeof string     string     string instanceof String                type    is          tests if  string       or      test string   is valid     else if            unacceptable to  amp  amp          string toString  amp  amp  typeof string toString     function   amp  amp  string toString    Object prototype toString               type    to          tests if      string   is valid     else if            unacceptable chars  amp  amp           string length  gt  0    string length    0                  type    chars             for each char         for  var index   0  type  amp  amp  index  lt  string length    index                var char   string index                  efficiently get its length             var length     duckyString char  false   to true                      char                   duckyString char  true                      length               if  length    1                  continue                 unicode surrogate-pair support             char   duckyString char  true               length   String prototype Symbol  amp  amp  Symbol iterator               if    length   length  amp  amp  length call char      length next   done     length next   done                  type   null                         return true or false if they dont want to auto-convert to real string     if    type  amp  amp  normalise             return truthy or falsy with  lt type gt  null if they want why it s true         return  normalise    null    type    null   type         perform conversion     switch  type        case  is           return string      case  to           return string toString        case  chars           return Array from string  join                Included are options to   ask which method deemed it string-y exclude methods of string-detection  eg if you dont like  toString      Here are more tests because I m a completionist   out  Edge-case testing   function test text  options        var result   duckyString text  false  options       text   duckyString text  true  options       out result           result          text         text      test      test null   test undefined   test 0   test  length 0    test   0       length  1     test      test window   test false   test   hi     test    uD83D uDE07     test    1    2  new String 3     test    1    2  new String 3     chars true       All negative cases seem to be accounted for This should run on browsers    IE8 Char arrays with multiple bytes supported on browsers with string iterator support   Output   Edge-case testing is    null null null null to  0  chars    chars     null null chars    to  false  null null chars    chars  123  to  1 2 3

User · Answer

if  s  amp  amp  typeof s valueOf        string          s is a string     Works for both string literals let s    blah  and for Object Strings let s   new String  blah

User · Answer

I also found that this works fine too  and its a lot shorter than the other examples   if  myVar     myVar              its string   else        its something else     By concatenating on empty quotes it turns the value into a string  If myVar is already a string then the if statement is successful

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You can use typeof operator   var booleanValue   true   var numericalValue   354  var stringValue    This is a String   var stringObject   new String   This is a String Object     alert typeof booleanValue     displays  boolean  alert typeof numericalValue     displays  number  alert typeof stringValue     displays  string  alert typeof stringObject     displays  object    Example from this webpage   Example was slightly modified though    This won t work as expected in the case of strings created with new String    but this is seldom used and recommended against 1  2   See the other answers for how to handle these  if you so desire      The Google JavaScript Style Guide says to never use primitive object wrappers  Douglas Crockford recommended that primitive object wrappers be deprecated

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Performance Today 2020 09 17 I perform tests on MacOs HighSierra 10 13 6 on Chrome v85  Safari v13 1 2 and Firefox v80 for chosen solutions  Results For all browsers  and both test cases   solutions typeof  instanceof  A  I  and x   x     H  are fast fastest solution   isString  lodash lib  is medium fast solutions B and K are slowest   Update  2020 11 28 I update results for x 123 Chrome column - for solution I there was probably an error value before   69M too low  - I use Chrome 86 0 to repeat tests  Details I perform 2 tests cases for solutions A B C D E F G H I J K L  when variable is string - you can run it HERE when variable is NOT string - you can run it HERE  Below snippet presents differences between solutions  x000D   x000D     https   stackoverflow com a 9436948 860099 function A x      return  typeof x     string       x instanceof String        https   stackoverflow com a 17772086 860099 function B x      return Object prototype toString call x        object String         https   stackoverflow com a 20958909 860099 function C x      return   isString x         https   stackoverflow com a 20958909 860099 function D x      return   type x       string         https   stackoverflow com a 16215800 860099 function E x      return x  constructor     String        https   stackoverflow com a 42493631 860099 function F x     return x  charAt    null        https   stackoverflow com a 57443488 860099 function G x     return String x      x       https   stackoverflow com a 19057360 860099 function H x     return x     x            https   stackoverflow com a 4059166 860099 function I x      return typeof x     string        https   stackoverflow com a 28722301 860099 function J x     return x     x  toString         https   stackoverflow com a 58892465 860099 function K x     return x  amp  amp  typeof x valueOf        string        https   stackoverflow com a 9436948 860099 function L x      return x instanceof String       ------------------        PRESENTATION    ------------------  console log  Solutions results for different inputs  n n    console log   abc  Str           1   0   1   0         true false null undef     let tests      abc   new String  abc           1   0  1 0       true false null undefined     A B C D E F G H I J K L  map f  gt      console log       f name        tests map v  gt   1   f v     join          x000D   lt script src  https   cdnjs cloudflare com ajax libs jquery 3 3 1 jquery min js  gt  lt  script gt   lt script src  https   cdnjs cloudflare com ajax libs lodash js 4 17 20 lodash min js  integrity  sha512-90vH1Z83AJY9DmlWa8WkjkV79yfS2n2Oxhsi2dZbIv0nC4E6m5AbH8Nh156kkM7JePmqD6tcZsfad1ueoaovww    crossorigin  anonymous  gt  lt  script gt    This shippet only presents functions used in performance tests - it not perform tests itself  x000D   x000D   x000D   And here are example results for chrome

User · Answer

This is good enough for me   WARNING  This is not a perfect solution  See the bottom of my post   Object prototype isString   function     return false     String prototype isString   function     return true      var isString   function a      return  a     null   amp  amp   a     undefined   amp  amp  a isString         And you can use this like below     return false isString null   isString void 0   isString -123   isString 0   isString true   isString false   isString      isString      isString function        isString 0 0      return true isString      isString new String  ABC       WARNING  This works incorrectly in the case     this is not a string var obj           but returns true lol     isString  function    return true       isString obj    should be false  but true

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var a   new String     var b      var c       function isString x      return x     null  amp  amp  x     undefined  amp  amp  x constructor     String    console log isString a   console log isString b   console log isString c

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The following method will check if any variable is a string  including variables that do not exist    const is string   value   gt      try       return typeof value        string       catch  error        return false          let example    Hello  world     console log is string      gt  example       true console log is string      gt  variable doesnt exist       false

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If you work on the node js environment  you can simply use the built-in function isString in utils   const util   require  util    if  util isString myVar        Edit  as  Jehy mentioned  this is deprecated since v4

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I can t honestly see why one would not simply use typeof in this case   if  typeof str      string       return 42      Yes it will fail against object-wrapped strings  e g  new String  foo    but these are widely regarded as a bad practice and most modern development tools are likely to discourage their use   If you see one  just fix it    The Object prototype toString trick is something that all front-end developers have been found guilty of doing one day in their careers but don t let it fool you by its polish of clever  it will break as soon as something monkey-patch the Object prototype    x000D   x000D  const isString   thing   gt  Object prototype toString call thing        object String    x000D   x000D  console log isString  foo     x000D   x000D  Object prototype toString        gt  42  x000D   x000D  console log isString  foo     x000D   x000D   x000D

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You can use this function to determine the type of anything  var type   function obj        return Object prototype toString apply obj  replace    object        i    1   toLowerCase        To check if a variable is a string  type  my string        string    true type new String  my string         string    true type  my string        string    true type 12345       string    false type          string     false  https   codepen io patodiblasi pen NQXPwY editors 0012 To check for other types  type null    null type undefined    undefined type       array type       object type function         function type 123    number type new Number 123     number type  some regex     regexp type Symbol  quot foo quot      symbol

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Just to expand on  DRAX s answer  I d do this   function isWhitespaceEmptyString str          RETURN                 true  if  str  is empty string  null  undefined  or consists of white-spaces only     return str       S  test str      str           str     null    str     undefined       It will account also for nulls and undefined types  and it will take care of non-string types  such as 0

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Since 580  people have voted for an incorrect answer  and 800  have voted for a working but shotgun-style answer  I thought it might be worth redoing my answer in a simpler form that everybody can understand   function isString x      return Object prototype toString call x        object String       Or  inline  I have an UltiSnip setup for this    Object prototype toString call myVar        object String     FYI  Pablo Santa Cruz s answer is wrong  because typeof new String  string   is object  DRAX s answer is accurate and functional  and should be the correct answer  since Pablo Santa Cruz is most definitely incorrect  and I won t argue against the popular vote    However  this answer is also definitely correct  and actually the best answer  except  perhaps  for the suggestion of using lodash underscore   disclaimer  I contributed to the lodash 4 codebase   My original answer  which obviously flew right over a lot of heads  follows    I transcoded this from underscore js     Arguments    Function    String    Number    Date    RegExp   forEach       function name             window  is    name    function obj                  return toString call obj       object     name                      That will define isString  isNumber  etc     In Node js  this can be implemented as a module   module exports        Arguments      Function       String       Number       Date       RegExp    reduce   obj  name    gt      obj   is    name     x   gt  toString call x       object     name          return obj             edit   Object prototype toString call x  works to delineate between functions and async functions as well    x000D   x000D  const fn1        gt  new Promise  resolve  reject    gt  setTimeout      gt  resolve      1000   x000D  const fn2   async      gt       x000D   x000D  console log  fn1   Object prototype toString call fn1   x000D  console log  fn2   Object prototype toString call fn2   x000D   x000D   x000D

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A Typechecker helper   function isFromType variable  type     if  typeof type     string   res    typeof variable    type toLowerCase      else res    variable constructor    type    return res     usage   isFromType  cs    string     true isFromType  cs   String    true isFromType   cs    Array    true isFromType   cs     object     false   Also if you want it to be recursive like Array that is an Object   you can use instanceof      cs   instanceof Object   true

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I find this simple technique useful to type-check for String -  String x      x    true  if x is a string                    false in every other case    x000D   x000D  const test   x   gt  x000D    console assert x000D        String x      x x000D         not a string    x   x000D        x000D   x000D  test  some string   x000D  test 123               assertion failed x000D  test 0                 assertion failed x000D  test  some regex       assertion failed x000D  test   5  6            assertion failed x000D  test   a  1            assertion failed x000D  test x   gt  x   1        assertion failed x000D   x000D   x000D    The same technique works for Number too -  Number x      x    true  if x is a number                    false in every other case    x000D   x000D  const test   x   gt  x000D    console assert x000D        Number x      x x000D         not a number    x   x000D        x000D   x000D  test  some string      assertion failed x000D  test 123             x000D  test 0               x000D  test  some regex       assertion failed x000D  test   5  6            assertion failed x000D  test   a  1            assertion failed x000D  test x   gt  x   1        assertion failed x000D   x000D   x000D    And for RegExp -  RegExp x      x    true  if x is a regexp                    false in every other case    x000D   x000D  const test   x   gt  x000D    console assert x000D        RegExp x      x x000D         not a regexp    x   x000D        x000D   x000D  test  some string      assertion failed x000D  test 123               assertion failed x000D  test 0                 assertion failed x000D  test  some regex     x000D  test   5  6            assertion failed x000D  test   a  1            assertion failed x000D  test x   gt  x   1        assertion failed x000D   x000D   x000D    Same for Object -  Object x      x    true  if x is an object                    false in every other case   NB  regexps  arrays  and functions are considered objects too    x000D   x000D  const test   x   gt  x000D    console assert x000D        Object x      x x000D         not an object    x   x000D        x000D   x000D  test  some string      assertion failed x000D  test 123               assertion failed x000D  test 0                 assertion failed x000D  test  some regex     x000D  test   5  6          x000D  test   a  1          x000D  test x   gt  x   1      x000D   x000D   x000D    But  checking for Array is a bit different -  Array isArray x      x    true  if x is an array                           false in every other case    x000D   x000D  const test   x   gt  x000D    console assert x000D        Array isArray x  x000D         not an array    x   x000D        x000D   x000D  test  some string      assertion failed x000D  test 123               assertion failed x000D  test 0                 assertion failed x000D  test  some regex       assertion failed x000D  test   5  6          x000D  test   a  1            assertion failed x000D  test x   gt  x   1        assertion failed x000D   x000D   x000D    This technique does not work for Functions however -  Function x      x    always false

User · Answer

This is a great example of why performance matters   Doing something as simple as a test for a string can be expensive if not done correctly   For example  if I wanted to write a function to test if something is a string  I could do it in one of two ways   1  const isString   str   gt   Object prototype toString call str        object String      2  const isString   str   gt    typeof str      string       str instanceof String     Both of these are pretty straight forward  so what could possibly impact performance   Generally speaking  function calls can be expensive  especially if you don t know what s happening inside  In the first example  there is a function call to Object s toString method   In the second example  there are no function calls  as typeof and instanceof are operators  Operators are significantly faster than function calls   When the performance is tested  example 1 is 79  slower than example 2   See the tests  https   jsperf com isstringtype

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I m not sure if you mean knowing if it s a type string regardless of its contents  or whether it s contents is a number or string  regardless of its type  So to know if its type is a string  that s already been answered   But to know based on its contents if its a string or a number  I would use this   function isNumber item        return  parseInt item            item      And for some examples   isNumber 123       true isNumber  123      true isNumber  123a     false isNumber           false

[javascript] Check if a variable is a string in JavaScript

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