Built-in way in JavaScript to check if a string is a valid number

Question

I m hoping there s something in the same conceptual space as the old VB6 IsNumeric   function

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I m using the following   const isNumber   s   gt   isNaN  s

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Quote      isNaN num             returns true if the variable does NOT contain a valid number   is not entirely true if you need to check for leading trailing spaces - for example when a certain quantity of digits is required  and you need to get  say   1111  and not   111  or  111   for perhaps a PIN input   Better to use   var num      d    test num

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Try the isNan function      The isNaN   function determines whether a value is an illegal number  Not-a-Number        This function returns true if the value equates to NaN  Otherwise it returns false       This function is different from the Number specific Number isNaN   method        nbsp  The global isNaN   function  converts the tested value to a Number  then tests it       Number isNan   does not convert the values to a Number  and will not return true for any value that is not of the type Number

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This is built on some of the previous answers and comments  The following covers all the edge cases and fairly concise as well   const isNumRegEx     -   d      d      function isNumeric n  allowScientificNotation   false        return allowScientificNotation                     Number isNaN parseFloat n    amp  amp  Number isFinite n                    isNumRegEx test n

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You could make use of types  like with the flow library  to get static  compile time checking   Of course not terribly useful for user input       flow  function acceptsNumber value  number                acceptsNumber 42            Works  acceptsNumber 3 14          Works  acceptsNumber NaN           Works  acceptsNumber Infinity      Works  acceptsNumber  foo          Error

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I do it like this   function isString value        return value length     undefined    function isNumber value        return value NaN     undefined      Of course isString   will be tripped up here if you pass some other object that has  length  defined

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And you could go the RegExp-way  var num    quot 987238 quot    if num match   -  d           valid integer  positive or negative   else if num match    d    d           valid float  else      not valid number

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This is built on some of the previous answers and comments  The following covers all the edge cases and fairly concise as well   const isNumRegEx     -   d      d      function isNumeric n  allowScientificNotation   false        return allowScientificNotation                     Number isNaN parseFloat n    amp  amp  Number isFinite n                    isNumRegEx test n

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The accepted answer for this question has quite a few flaws  as highlighted by couple of other users   This is one of the easiest  amp  proven way to approach it in javascript   function isNumeric n      return  isNaN parseFloat n    amp  amp  isFinite n       Below are some good test cases   console log isNumeric 12345678912345678912       true console log isNumeric  2                         true console log isNumeric  -32 2                     true console log isNumeric -32 2                      true console log isNumeric undefined                  false     the accepted answer fails at these tests  console log isNumeric                            false console log isNumeric null                       false console log isNumeric                            false

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And you could go the RegExp-way  var num    quot 987238 quot    if num match   -  d           valid integer  positive or negative   else if num match    d    d           valid float  else      not valid number

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My solution      returns true for positive ints      no scientific notation  hexadecimals or floating point dots  var isPositiveInt   function str        var result   true  chr     for  var i   0  n   str length  i  lt  n  i             chr   str charAt i          if   chr  lt   0     chr  gt   9    amp  amp  chr             not digit or thousands separator          result   false           break                   if  i    0  amp  amp   chr     0     chr               should not start with 0 or            result   false           break                     return result        You can add additional conditions inside the loop  to fit you particular needs

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function isNumberCandidate s      const str        s  trim      if  str length     0  return false    return  isNaN  str      console log isNumberCandidate  1              true console log isNumberCandidate  a              false console log isNumberCandidate  000            true console log isNumberCandidate  1a             false  console log isNumberCandidate  1e             false console log isNumberCandidate  1e-1           true console log isNumberCandidate  123 3          true console log isNumberCandidate                 false console log isNumberCandidate                 false console log isNumberCandidate 1               true console log isNumberCandidate 0               true console log isNumberCandidate NaN             false console log isNumberCandidate undefined       false console log isNumberCandidate null            false console log isNumberCandidate -1              true console log isNumberCandidate  -1             true console log isNumberCandidate  -1 2           true console log isNumberCandidate 0 0000001       true console log isNumberCandidate  0 0000001        true console log isNumberCandidate Infinity          true console log isNumberCandidate -Infinity          true  console log isNumberCandidate  Infinity         true  if  isNumberCandidate s          use  s as a number    s

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If anyone ever gets this far down  I spent some time hacking on this trying to patch moment js  https   github com moment moment   Here s something that I took away from it   function isNumeric val        var  val    val      return  val     val   1    infinity check          amp  amp    val      val    Cute coercion check          amp  amp   typeof val      object     Array object check     Handles the following cases   True     isNumeric  1    isNumeric 1e10   isNumeric 1E10   isNumeric   6e4    isNumeric  1 2222    isNumeric  -1 2222    isNumeric  -1 222200000000000000    isNumeric  1 222200000000000000    isNumeric 1   isNumeric 0   isNumeric -0   isNumeric 1010010293029   isNumeric 1 100393830000   isNumeric Math LN2   isNumeric Math PI   isNumeric 5e10     False     isNumeric NaN   isNumeric Infinity   isNumeric -Infinity   isNumeric    isNumeric undefined   isNumeric   1 2 3     isNumeric  a 1 b 2    isNumeric null   isNumeric  1    isNumeric new Date       Ironically  the one I am struggling with the most   isNumeric new Number 1     gt  false   Any suggestions welcome

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parseInt    but be aware that this function is a bit different in the sense that it for example returns 100 for parseInt  100px

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Well  I m using this one I made    It s been working so far  function checkNumber value        return value   1    0     If you spot any problem with it  tell me  please

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I like the simplicity of this  Number isNaN Number value    The above is regular Javascript  but I m using this in conjunction with a typescript typeguard for smart type checking  This is very useful for the typescript compiler to give you correct intellisense  and no type errors  Typescript typeguards isNotNumber value  string   number   value is string       return Number isNaN Number this smartImageWidth      isNumber value  string   number   value is number       return Number isNaN Number this smartImageWidth       false     Let s say you have a property width which is number   string  You may want to do logic based on whether or not it s a string  var width  number string  width    quot 100vw quot    if  isNotNumber width             the compiler knows that width here must be a string     if  width endsWith  vw                      we have a  width  such as 100vw          else           the compiler is smart and knows width here must be number     var doubleWidth   width   2         The typeguard is smart enough to constrain the type of width within the if statement to be ONLY string  This permits the compiler to allow width endsWith      which it wouldn t allow if the type was string   number  You can call the typeguard whatever you want isNotNumber  isNumber  isString  isNotString but I think isString is kind of ambiguous and harder to read

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2nd October 2020  note that many bare-bones approaches are fraught with subtle bugs  eg  whitespace  implicit partial parsing  radix  coercion of arrays etc   that many of the answers here fail to take into account  The following implementation might work for you  but note that it does not cater for number separators other than the decimal point  quot   quot   function isNumeric str      if  typeof str     quot string quot   return false    we only process strings      return  isNaN str   amp  amp     use type coercion to parse the  entirety  of the string   parseFloat  alone does not do this               isNaN parseFloat str         and ensure strings of whitespace fail     To check if a variable  including a string  is a number  check if it is not a number  This works regardless of whether the variable content is a string or number  isNaN num             returns true if the variable does NOT contain a valid number  Examples isNaN 123             false isNaN  123            false isNaN  1e10000        false  This translates to Infinity  which is a number  isNaN  foo            true isNaN  10px           true  Of course  you can negate this if you need to  For example  to implement the IsNumeric example you gave  function isNumeric num     return  isNaN num     To convert a string containing a number into a number  Only works if the string only contains numeric characters  else it returns NaN   num                  returns the numeric value of the string  or NaN                        if the string isn t purely numeric characters  Examples   12                  12   12                  12   12                  NaN    12                 0 12     12                NaN   foo                 NaN   12px                NaN  To convert a string loosely to a number Useful for converting  12px  to 12  for example  parseInt num          extracts a numeric value from the                        start of the string  or NaN   Examples parseInt  12          12 parseInt  aaa         NaN parseInt  12px        12 parseInt  foo2        NaN      These last two may be different parseInt  12a5        12       from what you expected to see    Floats Bear in mind that  unlike  num  parseInt  as the name suggests  will convert a float into an integer by chopping off everything following the decimal point  if you want to use parseInt   because of this behaviour  you re probably better off using another method instead     12 345              12 345 parseInt 12 345       12 parseInt  12 345      12  Empty strings Empty strings may be a little counter-intuitive   num converts empty strings or strings with spaces to zero  and isNaN   assumes the same                        0                       0 isNaN                 false isNaN                 false  But parseInt   does not agree  parseInt              NaN parseInt              NaN

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Try the isNan function      The isNaN   function determines whether a value is an illegal number  Not-a-Number        This function returns true if the value equates to NaN  Otherwise it returns false       This function is different from the Number specific Number isNaN   method        nbsp  The global isNaN   function  converts the tested value to a Number  then tests it       Number isNan   does not convert the values to a Number  and will not return true for any value that is not of the type Number

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In my application we are only allowing a-z A-Z and 0-9 characters  I found the answer above using   string   1     0  worked unless the string began with 0xnn  like 0x10  and then it would return it as numeric when we didn t want it to  The following simple trap in my numeric check seems to do the trick in our specific cases       function isStringNumeric str input            concat a temporary 1 during the modulus to keep a beginning hex switch combination from messing us up          very simple and as long as special characters  non a-z A-Z 0-9  are trapped it is fine        return  1  concat str input    1     0     Warning   This might be exploiting a longstanding bug in Javascript and Actionscript  Number  1    the string    1     0     I can t speak for that  but it is exactly what we needed

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You could make use of types  like with the flow library  to get static  compile time checking   Of course not terribly useful for user input       flow  function acceptsNumber value  number                acceptsNumber 42            Works  acceptsNumber 3 14          Works  acceptsNumber NaN           Works  acceptsNumber Infinity      Works  acceptsNumber  foo          Error

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2019  Including ES3  ES6 and TypeScript Examples  Maybe this has been rehashed too many times  however I fought with this one today too and wanted to post my answer  as I didn t see any other answer that does it as simply or thoroughly   ES3  var isNumeric   function num       return  typeof num       number     typeof num       string   amp  amp  num trim            amp  amp   isNaN num         ES6  const isNumeric    num    gt   typeof num       number     typeof num       string   amp  amp  num trim            amp  amp   isNaN num     Typescript  const isNumeric    num  any    gt   typeof num       number     typeof num       string   amp  amp  num trim            amp  amp   isNaN num as number     This seems quite simple and covers all the bases I saw on the many other posts and thought up myself      Positive Cases console log 0  isNumeric 0      true   console log 1  isNumeric 1      true   console log 1234567890  isNumeric 1234567890      true   console log  1234567890   isNumeric  1234567890       true   console log  0   isNumeric  0       true   console log  1   isNumeric  1       true   console log  1 1   isNumeric  1 1       true   console log  -1   isNumeric  -1       true   console log  -1 2354   isNumeric  -1 2354       true   console log  -1234567890   isNumeric  -1234567890       true   console log -1  isNumeric -1      true   console log -32 1  isNumeric -32 1      true   console log  0x1   isNumeric  0x1       true       Valid number in hex    Negative Cases console log true  isNumeric true      false   console log false  isNumeric false      false   console log  1  1   isNumeric  1  1       false   console log  1 1   isNumeric  1 1       false   console log  -32 1 12   isNumeric  -32 1 12       false   console log   blank    isNumeric         false   console log   spaces    isNumeric            false   console log  null   isNumeric null      false   console log  undefined   isNumeric undefined      false   console log     isNumeric         false   console log  NaN   isNumeric NaN      false     You can also try your own isNumeric function and just past in these use cases and scan for  true  for all of them   Or  to see the values that each return

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function isNumberCandidate s      const str        s  trim      if  str length     0  return false    return  isNaN  str      console log isNumberCandidate  1              true console log isNumberCandidate  a              false console log isNumberCandidate  000            true console log isNumberCandidate  1a             false  console log isNumberCandidate  1e             false console log isNumberCandidate  1e-1           true console log isNumberCandidate  123 3          true console log isNumberCandidate                 false console log isNumberCandidate                 false console log isNumberCandidate 1               true console log isNumberCandidate 0               true console log isNumberCandidate NaN             false console log isNumberCandidate undefined       false console log isNumberCandidate null            false console log isNumberCandidate -1              true console log isNumberCandidate  -1             true console log isNumberCandidate  -1 2           true console log isNumberCandidate 0 0000001       true console log isNumberCandidate  0 0000001        true console log isNumberCandidate Infinity          true console log isNumberCandidate -Infinity          true  console log isNumberCandidate  Infinity         true  if  isNumberCandidate s          use  s as a number    s

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2019  Practical and tight numerical validity check  Often  a  valid number  means a Javascript number excluding NaN and Infinity  ie a  finite number    To check the numerical validity of a value  from an external source for example   you can define in ESlint Airbnb style           Returns true if  candidate  is a finite number or a string referring  not just  including   a finite number    To keep in mind       Number true    1      Number       0      Number     10       10       isNaN true    true      parseFloat  10 a     10        param     candidate     return  boolean      function isReferringFiniteNumber candidate      if  typeof  candidate       number   return Number isFinite candidate     if  typeof  candidate       string         return  candidate trim            amp  amp  Number isFinite Number candidate          return false      and use it this way   if  isReferringFiniteNumber theirValue       myCheckedValue   Number theirValue     else     console warn  The provided value doesn  t refer to a finite number

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This appears to catch the seemingly infinite number of edge cases   function isNumber x  noStr                    - Returns true if x is either a finite number type or a string containing only a number         - If empty string supplied  fall back to explicit false         - Pass true for noStr to return false when typeof x is  string   off by default          isNumber       false         isNumber         false         isNumber  1       false         isNumber  1 2       false         isNumber         false         isNumber null      false         isNumber         false         isNumber true      false         isNumber  true       false         isNumber  false       false         isNumber  123asdf       false         isNumber  123 asdf       false         isNumber undefined      false         isNumber Number POSITIVE INFINITY      false         isNumber Number NEGATIVE INFINITY      false         isNumber  Infinity       false         isNumber  -Infinity       false         isNumber Number NaN      false         isNumber new Date  December 17  1995 03 24 00        false         isNumber 0      true         isNumber  0       true         isNumber 123      true         isNumber 123 456      true         isNumber -123 456      true         isNumber - 123456      true         isNumber  123       true         isNumber  123 456       true         isNumber   123       true         isNumber  123      true         isNumber Number MAX SAFE INTEGER      true         isNumber Number MAX VALUE      true         isNumber Number MIN VALUE      true         isNumber new Number 123       true             return            typeof x      number     x instanceof Number      noStr  amp  amp  x  amp  amp  typeof x      string   amp  amp   isNaN x     amp  amp          isFinite x           false

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My attempt at a slightly confusing  Pherhaps not the best solution  function isInt a       return a          a     console log isInt  abcd                false console log isInt  123a                false console log isInt  1                   true console log isInt  0                   true console log isInt  -0                  false console log isInt  01                  false console log isInt  10                  true console log isInt  -1234567890         true console log isInt 1234                 false console log isInt  123 4               false console log isInt                      false     other types then string returns false console log isInt 5                    false console log isInt undefined            false console log isInt null                 false console log isInt  0x1                 false console log isInt Infinity             false

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It is not valid for TypeScript as   declare function isNaN number  number   boolean   For TypeScript you can use      d    test key

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parseInt    but be aware that this function is a bit different in the sense that it for example returns 100 for parseInt  100px

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Maybe there are one or two people coming across this question who need a much stricter check than usual  like I did   In that case  this might be useful   if str     String Number str           it s a  perfectly formatted  number     Beware  This will reject strings like  1  40 000  080  00 1  It s very picky - the string must match the  most minimal perfect form  of the number for this test to pass   It uses the String and Number constructor to cast the string to a number and back again and thus checks if the JavaScript engine s  perfect minimal form   the one it got converted to with the initial Number constructor  matches the original string

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Well  I m using this one I made    It s been working so far  function checkNumber value        return value   1    0     If you spot any problem with it  tell me  please

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Old question  but there are several points missing in the given answers   Scientific notation    isNaN  1e 30   is true  however in most of the cases when people ask for numbers  they do not want to match things like 1e 30   Large floating numbers may behave weird  Observe  using Node js     gt  var s   Array 16   1  join  9   undefined  gt  s length 16  gt  s  9999999999999999   gt   isNaN s  true  gt  Number s  10000000000000000  gt  String Number s       s false  gt    On the other hand    gt  var s   Array 16   1  join  1   undefined  gt  String Number s       s true  gt  var s   Array 15   1  join  9   undefined  gt  String Number s       s true  gt    So  if one expects String Number s       s  then better limit your strings to 15 digits at most  after omitting leading zeros    Infinity   gt  typeof Infinity  number   gt   isNaN  Infinity   true  gt  isFinite  Infinity   false  gt    Given all that  checking that the given string is a number satisfying all of the following    non scientific notation predictable conversion to Number and back to String finite   is not such an easy task  Here is a simple version     function isNonScientificNumberString o        if   o    typeof o      string              Should not be given anything but strings        return false            return o length  lt   15  amp  amp  o indexOf  e     lt  0  amp  amp  o indexOf  E     lt  0  amp  amp   isNaN o   amp  amp  isFinite o         However  even this one is far from complete  Leading zeros are not handled here  but they do screw the length test

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Why is jQuery s implementation not good enough   function isNumeric a        var b   a  amp  amp  a toString        return    isArray a   amp  amp  b - parseFloat b    1  gt   0       Michael suggested something like this  although I ve stolen  user1691651 - John  s altered version here    function isNumeric num       num        num    coerce num to be a string     return  isNaN num   amp  amp   isNaN parseFloat num        The following is a solution with most likely bad performance  but solid results  It is a contraption made from the jQuery 1 12 4 implementation and Michael s answer  with an extra check for leading trailing spaces  because Michael s version returns true for numerics with leading trailing spaces    function isNumeric a        var str   a           var b   a  amp  amp  a toString        return    isArray a   amp  amp  b - parseFloat b    1  gt   0  amp  amp                 s   s   g test str   amp  amp              isNaN str   amp  amp   isNaN parseFloat str         The latter version has two new variables  though  One could get around one of those  by doing   function isNumeric a        if    isArray a   return false      var b   a  amp  amp  a toString        a   a           return b - parseFloat b    1  gt   0  amp  amp                  s   s   g test a   amp  amp               isNaN a   amp  amp   isNaN parseFloat a         I haven t tested any of these very much  by other means than manually testing the few use-cases I ll be hitting with my current predicament  which is all very standard stuff  This is a  standing-on-the-shoulders-of-giants  situation

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Maybe there are one or two people coming across this question who need a much stricter check than usual  like I did   In that case  this might be useful   if str     String Number str           it s a  perfectly formatted  number     Beware  This will reject strings like  1  40 000  080  00 1  It s very picky - the string must match the  most minimal perfect form  of the number for this test to pass   It uses the String and Number constructor to cast the string to a number and back again and thus checks if the JavaScript engine s  perfect minimal form   the one it got converted to with the initial Number constructor  matches the original string

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Here is a high-performance  2 5 10 7 iterations s  3 8GHz Haswell  version of a isNumber implementation  It works for every testcase i could find  including Symbols    var isNumber    function        var isIntegerTest      d       var isDigitArray     0   0   0   0   0   0   0   0   0   0     function hasLeading0s  s        return   typeof s      string         s length  lt  2        s 0       0          isDigitArray s 1          isIntegerTest test s          var isWhiteSpaceTest     s     return function isNumber  s        var t   typeof s      var n      if  t      number           return  s  lt   0      s  gt  0         else if  t      string           n    s        return      n  lt   0   amp  amp    n  gt  0      n      0     hasLeading0s s       n     0      s           isWhiteSpaceTest test s            else if  t      object           return     s instanceof Number       n    s     n  lt   0   amp  amp    n  gt  0               return false

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parseInt    but be aware that this function is a bit different in the sense that it for example returns 100 for parseInt  100px

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And you could go the RegExp-way  var num    quot 987238 quot    if num match   -  d           valid integer  positive or negative   else if num match    d    d           valid float  else      not valid number

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If you re just trying to check if a string is a whole number  no decimal places   regex is a good way to go  Other methods such as isNaN are too complicated for something so simple      function isNumeric value            return   -  d    test value                  console log isNumeric  abcd                false     console log isNumeric  123a                false     console log isNumeric  1                   true     console log isNumeric  1234567890          true     console log isNumeric  -23                 true     console log isNumeric 1234                 true     console log isNumeric  123 4               false     console log isNumeric                      false     console log isNumeric undefined            false     console log isNumeric null                 false  To only allow positive whole numbers use this      function isNumeric value            return    d    test value              console log isNumeric  123                 true     console log isNumeric  -23                 false

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You can use the result of Number when passing an argument to its constructor   If the argument  a string  cannot be converted into a number  it returns NaN  so you can determinate if the string provided was a valid number or not   Notes  Note when passing empty string or    t t  and   n t  as Number will return 0  Passing true will return 1 and false returns 0       Number  34 00      34     Number  -34      -34     Number  123e5      12300000     Number  123e-5      0 00123     Number  999999999999      999999999999     Number  9999999999999999      10000000000000000  integer accuracy up to 15 digit      Number  0xFF      255     Number  Infinity      Infinity        Number  34px      NaN     Number  xyz      NaN     Number  true      NaN     Number  false      NaN         cavets     Number            0     Number   t t      0     Number   n t      0

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PFB the working solution    function check        check   check           var isNumber     check trim   length gt 0   isNaN check  false      return isNumber

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I recently wrote an article about ways to ensure a variable is a valid number  https   github com jehugaleahsa artifacts blob master 2018 typescript num hack md The article explains how to ensure floating point or integer  if that s important   x vs   x    The article assumes the variable is a string or a number to begin with and trim is available polyfilled  It wouldn t be hard to extend it to handle other types  as well  Here s the meat of it      Check for a valid float if  x    null              x  trim                 isNaN  x         return false      not a float       Check for a valid integer if  x    null              x  trim                   x      x        return false      not an integer

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I m using the following   const isNumber   s   gt   isNaN  s

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In my application we are only allowing a-z A-Z and 0-9 characters  I found the answer above using   string   1     0  worked unless the string began with 0xnn  like 0x10  and then it would return it as numeric when we didn t want it to  The following simple trap in my numeric check seems to do the trick in our specific cases       function isStringNumeric str input            concat a temporary 1 during the modulus to keep a beginning hex switch combination from messing us up          very simple and as long as special characters  non a-z A-Z 0-9  are trapped it is fine        return  1  concat str input    1     0     Warning   This might be exploiting a longstanding bug in Javascript and Actionscript  Number  1    the string    1     0     I can t speak for that  but it is exactly what we needed

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My attempt at a slightly confusing  Pherhaps not the best solution  function isInt a       return a          a     console log isInt  abcd                false console log isInt  123a                false console log isInt  1                   true console log isInt  0                   true console log isInt  -0                  false console log isInt  01                  false console log isInt  10                  true console log isInt  -1234567890         true console log isInt 1234                 false console log isInt  123 4               false console log isInt                      false     other types then string returns false console log isInt 5                    false console log isInt undefined            false console log isInt null                 false console log isInt  0x1                 false console log isInt Infinity             false

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Save yourself the headache of trying to find a  built-in  solution   There isn t a good answer  and the hugely upvoted answer in this thread is wrong   npm install is-number     In JavaScript  it s not always as straightforward as it should be to reliably check if a value is a number  It s common for devs to use    -  or Number   to cast a string value to a number  for example  when values are returned from user input  regex matches  parsers  etc   But there are many non-intuitive edge cases that yield unexpected results    console log           gt  0 console log           gt  0 console log              gt  0 console log typeof NaN       gt   number

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It is not valid for TypeScript as   declare function isNaN number  number   boolean   For TypeScript you can use      d    test key

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2019  Practical and tight numerical validity check  Often  a  valid number  means a Javascript number excluding NaN and Infinity  ie a  finite number    To check the numerical validity of a value  from an external source for example   you can define in ESlint Airbnb style           Returns true if  candidate  is a finite number or a string referring  not just  including   a finite number    To keep in mind       Number true    1      Number       0      Number     10       10       isNaN true    true      parseFloat  10 a     10        param     candidate     return  boolean      function isReferringFiniteNumber candidate      if  typeof  candidate       number   return Number isFinite candidate     if  typeof  candidate       string         return  candidate trim            amp  amp  Number isFinite Number candidate          return false      and use it this way   if  isReferringFiniteNumber theirValue       myCheckedValue   Number theirValue     else     console warn  The provided value doesn  t refer to a finite number

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Why is jQuery s implementation not good enough   function isNumeric a        var b   a  amp  amp  a toString        return    isArray a   amp  amp  b - parseFloat b    1  gt   0       Michael suggested something like this  although I ve stolen  user1691651 - John  s altered version here    function isNumeric num       num        num    coerce num to be a string     return  isNaN num   amp  amp   isNaN parseFloat num        The following is a solution with most likely bad performance  but solid results  It is a contraption made from the jQuery 1 12 4 implementation and Michael s answer  with an extra check for leading trailing spaces  because Michael s version returns true for numerics with leading trailing spaces    function isNumeric a        var str   a           var b   a  amp  amp  a toString        return    isArray a   amp  amp  b - parseFloat b    1  gt   0  amp  amp                 s   s   g test str   amp  amp              isNaN str   amp  amp   isNaN parseFloat str         The latter version has two new variables  though  One could get around one of those  by doing   function isNumeric a        if    isArray a   return false      var b   a  amp  amp  a toString        a   a           return b - parseFloat b    1  gt   0  amp  amp                  s   s   g test a   amp  amp               isNaN a   amp  amp   isNaN parseFloat a         I haven t tested any of these very much  by other means than manually testing the few use-cases I ll be hitting with my current predicament  which is all very standard stuff  This is a  standing-on-the-shoulders-of-giants  situation

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I recently wrote an article about ways to ensure a variable is a valid number  https   github com jehugaleahsa artifacts blob master 2018 typescript num hack md The article explains how to ensure floating point or integer  if that s important   x vs   x    The article assumes the variable is a string or a number to begin with and trim is available polyfilled  It wouldn t be hard to extend it to handle other types  as well  Here s the meat of it      Check for a valid float if  x    null              x  trim                 isNaN  x         return false      not a float       Check for a valid integer if  x    null              x  trim                   x      x        return false      not an integer

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And you could go the RegExp-way  var num    quot 987238 quot    if num match   -  d           valid integer  positive or negative   else if num match    d    d           valid float  else      not valid number

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I used this function as a form validation tool  and I didn t want users to be able to write exponential function  so I came up with this function   lt script gt       function isNumber value  acceptScientificNotation             if true     acceptScientificNotation               return   - 0 1  d     d      test value                      if  true     Array isArray value                 return false                    return  isNaN parseInt value  10                console log isNumber  quot  quot                     false     console log isNumber false                 false     console log isNumber true                  false     console log isNumber  quot 0 quot                    true     console log isNumber  quot 0 1 quot                  true     console log isNumber  quot 12 quot                   true     console log isNumber  quot -12 quot                  true     console log isNumber -45                   true     console log isNumber  jo   quot pi quot              false     console log isNumber                       false     console log isNumber  78  79               false     console log isNumber NaN                   false     console log isNumber Infinity              false     console log isNumber undefined             false     console log isNumber  quot 0 1 quot                  false        console log isNumber  quot 1e-1 quot                 false     console log isNumber  quot 1e-1 quot   true          true  lt  script gt

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When guarding against empty strings and null    Base cases that are handled properly Number isNaN Number  1          gt  false Number isNaN Number  -1          gt  false Number isNaN Number  1 1          gt  false Number isNaN Number  -1 1          gt  false Number isNaN Number  asdf          gt  true Number isNaN Number undefined         gt  true     Special notation cases that are handled properly Number isNaN Number  1e1          gt  false Number isNaN Number  1e-1          gt  false Number isNaN Number  -1e1          gt  false Number isNaN Number  -1e-1          gt  false Number isNaN Number  0b1          gt  false Number isNaN Number  0o1          gt  false Number isNaN Number  0xa          gt  false     Edge cases that will FAIL if not guarded against Number isNaN Number            gt  false Number isNaN Number             gt  false Number isNaN Number null         gt  false     Edge cases that are debatable Number isNaN Number  -0b1          gt  true Number isNaN Number  -0o1          gt  true Number isNaN Number  -0xa          gt  true Number isNaN Number  Infinity          gt  false  Number isNaN Number  INFINITY          gt  true   Number isNaN Number  -Infinity          gt  false  Number isNaN Number  -INFINITY          gt  true    When NOT guarding against empty strings and null Using parseInt     Base cases that are handled properly Number isNaN parseInt  1          gt  false Number isNaN parseInt  -1          gt  false Number isNaN parseInt  1 1          gt  false Number isNaN parseInt  -1 1          gt  false Number isNaN parseInt  asdf          gt  true Number isNaN parseInt undefined         gt  true Number isNaN parseInt            gt  true Number isNaN parseInt             gt  true Number isNaN parseInt null         gt  true     Special notation cases that are handled properly Number isNaN parseInt  1e1          gt  false Number isNaN parseInt  1e-1          gt  false Number isNaN parseInt  -1e1          gt  false Number isNaN parseInt  -1e-1          gt  false Number isNaN parseInt  0b1          gt  false Number isNaN parseInt  0o1          gt  false Number isNaN parseInt  0xa          gt  false     Edge cases that are debatable Number isNaN parseInt  -0b1          gt  false Number isNaN parseInt  -0o1          gt  false Number isNaN parseInt  -0xa          gt  false Number isNaN parseInt  Infinity          gt  true  Number isNaN parseInt  INFINITY          gt  true   Number isNaN parseInt  -Infinity          gt  true  Number isNaN parseInt  -INFINITY          gt  true   Using parseFloat     Base cases that are handled properly Number isNaN parseFloat  1          gt  false Number isNaN parseFloat  -1          gt  false Number isNaN parseFloat  1 1          gt  false Number isNaN parseFloat  -1 1          gt  false Number isNaN parseFloat  asdf          gt  true Number isNaN parseFloat undefined         gt  true Number isNaN parseFloat            gt  true Number isNaN parseFloat             gt  true Number isNaN parseFloat null         gt  true     Special notation cases that are handled properly Number isNaN parseFloat  1e1          gt  false Number isNaN parseFloat  1e-1          gt  false Number isNaN parseFloat  -1e1          gt  false Number isNaN parseFloat  -1e-1          gt  false Number isNaN parseFloat  0b1          gt  false Number isNaN parseFloat  0o1          gt  false Number isNaN parseFloat  0xa          gt  false     Edge cases that are debatable Number isNaN parseFloat  -0b1          gt  false Number isNaN parseFloat  -0o1          gt  false Number isNaN parseFloat  -0xa          gt  false Number isNaN parseFloat  Infinity          gt  false  Number isNaN parseFloat  INFINITY          gt  true   Number isNaN parseFloat  -Infinity          gt  false  Number isNaN parseFloat  -INFINITY          gt  true  Notes   Only string  empty  and uninitialized values are considered in keeping with addressing the original question   Additional edge cases exist if arrays and objects are the values being considered  Characters in binary  octal  hexadecimal  and exponential notation are not case-sensitive  ie   0xFF    0XFF    0xfF  etc  will all yield the same result in the test cases shown above   Unlike with Infinity  case-sensitive  in some cases  constants from the Number and Math objects passed as test cases in string format to any of the methods above will be determined to not be numbers  See here for an explanation of how arguments are converted to a Number and why the edge cases for null and empty strings exist

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2nd October 2020  note that many bare-bones approaches are fraught with subtle bugs  eg  whitespace  implicit partial parsing  radix  coercion of arrays etc   that many of the answers here fail to take into account  The following implementation might work for you  but note that it does not cater for number separators other than the decimal point  quot   quot   function isNumeric str      if  typeof str     quot string quot   return false    we only process strings      return  isNaN str   amp  amp     use type coercion to parse the  entirety  of the string   parseFloat  alone does not do this               isNaN parseFloat str         and ensure strings of whitespace fail     To check if a variable  including a string  is a number  check if it is not a number  This works regardless of whether the variable content is a string or number  isNaN num             returns true if the variable does NOT contain a valid number  Examples isNaN 123             false isNaN  123            false isNaN  1e10000        false  This translates to Infinity  which is a number  isNaN  foo            true isNaN  10px           true  Of course  you can negate this if you need to  For example  to implement the IsNumeric example you gave  function isNumeric num     return  isNaN num     To convert a string containing a number into a number  Only works if the string only contains numeric characters  else it returns NaN   num                  returns the numeric value of the string  or NaN                        if the string isn t purely numeric characters  Examples   12                  12   12                  12   12                  NaN    12                 0 12     12                NaN   foo                 NaN   12px                NaN  To convert a string loosely to a number Useful for converting  12px  to 12  for example  parseInt num          extracts a numeric value from the                        start of the string  or NaN   Examples parseInt  12          12 parseInt  aaa         NaN parseInt  12px        12 parseInt  foo2        NaN      These last two may be different parseInt  12a5        12       from what you expected to see    Floats Bear in mind that  unlike  num  parseInt  as the name suggests  will convert a float into an integer by chopping off everything following the decimal point  if you want to use parseInt   because of this behaviour  you re probably better off using another method instead     12 345              12 345 parseInt 12 345       12 parseInt  12 345      12  Empty strings Empty strings may be a little counter-intuitive   num converts empty strings or strings with spaces to zero  and isNaN   assumes the same                        0                       0 isNaN                 false isNaN                 false  But parseInt   does not agree  parseInt              NaN parseInt              NaN

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I have tested and Michael s solution is best  Vote for his answer above  search this page for  If you really want to make sure that a string  to find it   In essence  his answer is this   function isNumeric num     num        num    coerce num to be a string   return  isNaN num   amp  amp   isNaN parseFloat num        It works for every test case  which I documented here  https   jsfiddle net wggehvp9 5   Many of the other solutions fail for these edge cases       null      true  and     In theory  you could use them  with proper error handling  for example   return  isNaN num     or  return   num      num     with special handling for    s   null      true  false      and others

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PFB the working solution    function check        check   check           var isNumber     check trim   length gt 0   isNaN check  false      return isNumber

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Here s a one-liner to check if sNum is a valid numeric value  it has been tested for a wide variety of inputs    isNaN  s replace   s    g            returns True if numeric value

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I have tested and Michael s solution is best  Vote for his answer above  search this page for  If you really want to make sure that a string  to find it   In essence  his answer is this   function isNumeric num     num        num    coerce num to be a string   return  isNaN num   amp  amp   isNaN parseFloat num        It works for every test case  which I documented here  https   jsfiddle net wggehvp9 5   Many of the other solutions fail for these edge cases       null      true  and     In theory  you could use them  with proper error handling  for example   return  isNaN num     or  return   num      num     with special handling for    s   null      true  false      and others

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parseInt    but be aware that this function is a bit different in the sense that it for example returns 100 for parseInt  100px

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My solution      returns true for positive ints      no scientific notation  hexadecimals or floating point dots  var isPositiveInt   function str        var result   true  chr     for  var i   0  n   str length  i  lt  n  i             chr   str charAt i          if   chr  lt   0     chr  gt   9    amp  amp  chr             not digit or thousands separator          result   false           break                   if  i    0  amp  amp   chr     0     chr               should not start with 0 or            result   false           break                     return result        You can add additional conditions inside the loop  to fit you particular needs

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If you really want to make sure that a string contains only a number  any number  integer or floating point   and exactly a number  you cannot use parseInt    parseFloat    Number    or  isNaN   by themselves   Note that  isNaN   is actually returning true when Number   would return a number  and false when it would return NaN  so I will exclude it from the rest of the discussion   The problem with parseFloat   is that it will return a number if the string contains any number  even if the string doesn t contain only and exactly a number   parseFloat  2016-12-31       returns 2016 parseFloat  1-1      return 1 parseFloat  1 2 3      returns 1 2   The problem with Number   is that it will return a number in cases where the passed value is not a number at all   Number        returns 0 Number         returns 0 Number    u00A0    t n r      returns 0   The problem with rolling your own regex is that unless you create the exact regex for matching a floating point number as Javascript recognizes it you are going to miss cases or recognize cases where you shouldn t   And even if you can roll your own regex  why   There are simpler built-in ways to do it   However  it turns out that Number    and isNaN    does the right thing for every case where parseFloat   returns a number when it shouldn t  and vice versa   So to find out if a string is really exactly and only a number  call both functions and see if they both return true   function isNumber str      if  typeof str     string   return false    we only process strings       could also coerce to string  str      str   return  isNaN str   amp  amp   isNaN parseFloat str

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You can use the result of Number when passing an argument to its constructor   If the argument  a string  cannot be converted into a number  it returns NaN  so you can determinate if the string provided was a valid number or not   Notes  Note when passing empty string or    t t  and   n t  as Number will return 0  Passing true will return 1 and false returns 0       Number  34 00      34     Number  -34      -34     Number  123e5      12300000     Number  123e-5      0 00123     Number  999999999999      999999999999     Number  9999999999999999      10000000000000000  integer accuracy up to 15 digit      Number  0xFF      255     Number  Infinity      Infinity        Number  34px      NaN     Number  xyz      NaN     Number  true      NaN     Number  false      NaN         cavets     Number            0     Number   t t      0     Number   n t      0

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Try the isNan function      The isNaN   function determines whether a value is an illegal number  Not-a-Number        This function returns true if the value equates to NaN  Otherwise it returns false       This function is different from the Number specific Number isNaN   method        nbsp  The global isNaN   function  converts the tested value to a Number  then tests it       Number isNan   does not convert the values to a Number  and will not return true for any value that is not of the type Number

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If you really want to make sure that a string contains only a number  any number  integer or floating point   and exactly a number  you cannot use parseInt    parseFloat    Number    or  isNaN   by themselves   Note that  isNaN   is actually returning true when Number   would return a number  and false when it would return NaN  so I will exclude it from the rest of the discussion   The problem with parseFloat   is that it will return a number if the string contains any number  even if the string doesn t contain only and exactly a number   parseFloat  2016-12-31       returns 2016 parseFloat  1-1      return 1 parseFloat  1 2 3      returns 1 2   The problem with Number   is that it will return a number in cases where the passed value is not a number at all   Number        returns 0 Number         returns 0 Number    u00A0    t n r      returns 0   The problem with rolling your own regex is that unless you create the exact regex for matching a floating point number as Javascript recognizes it you are going to miss cases or recognize cases where you shouldn t   And even if you can roll your own regex  why   There are simpler built-in ways to do it   However  it turns out that Number    and isNaN    does the right thing for every case where parseFloat   returns a number when it shouldn t  and vice versa   So to find out if a string is really exactly and only a number  call both functions and see if they both return true   function isNumber str      if  typeof str     string   return false    we only process strings       could also coerce to string  str      str   return  isNaN str   amp  amp   isNaN parseFloat str

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So  it will depend on the test cases that you want it to handle  function isNumeric number      return  isNaN parseFloat number    amp  amp   isNaN  number      What I was looking for was regular types of numbers in javascript  0  1   -1  1 1   -1 1   1E1   -1E1   1e1   -1e1  0 1e10  -0 1 e10   0xAF1   0o172  Math PI  Number NEGATIVE INFINITY  Number POSITIVE INFINITY And also they re representations as strings   0    1    -1    1 1    -1 1    1E1    -1E1    1e1    -1e1    0 1e10    -0 1 e10    0xAF1    0o172  I did want to leave out and not mark them as numeric                   null  undefined  NaN As of today  all other answers seemed to failed one of these test cases

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Just use isNaN    this will convert the string to a number and if get a valid number  will return false     isNaN  Alireza      return true isNaN  123      return false

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When guarding against empty strings and null    Base cases that are handled properly Number isNaN Number  1          gt  false Number isNaN Number  -1          gt  false Number isNaN Number  1 1          gt  false Number isNaN Number  -1 1          gt  false Number isNaN Number  asdf          gt  true Number isNaN Number undefined         gt  true     Special notation cases that are handled properly Number isNaN Number  1e1          gt  false Number isNaN Number  1e-1          gt  false Number isNaN Number  -1e1          gt  false Number isNaN Number  -1e-1          gt  false Number isNaN Number  0b1          gt  false Number isNaN Number  0o1          gt  false Number isNaN Number  0xa          gt  false     Edge cases that will FAIL if not guarded against Number isNaN Number            gt  false Number isNaN Number             gt  false Number isNaN Number null         gt  false     Edge cases that are debatable Number isNaN Number  -0b1          gt  true Number isNaN Number  -0o1          gt  true Number isNaN Number  -0xa          gt  true Number isNaN Number  Infinity          gt  false  Number isNaN Number  INFINITY          gt  true   Number isNaN Number  -Infinity          gt  false  Number isNaN Number  -INFINITY          gt  true    When NOT guarding against empty strings and null Using parseInt     Base cases that are handled properly Number isNaN parseInt  1          gt  false Number isNaN parseInt  -1          gt  false Number isNaN parseInt  1 1          gt  false Number isNaN parseInt  -1 1          gt  false Number isNaN parseInt  asdf          gt  true Number isNaN parseInt undefined         gt  true Number isNaN parseInt            gt  true Number isNaN parseInt             gt  true Number isNaN parseInt null         gt  true     Special notation cases that are handled properly Number isNaN parseInt  1e1          gt  false Number isNaN parseInt  1e-1          gt  false Number isNaN parseInt  -1e1          gt  false Number isNaN parseInt  -1e-1          gt  false Number isNaN parseInt  0b1          gt  false Number isNaN parseInt  0o1          gt  false Number isNaN parseInt  0xa          gt  false     Edge cases that are debatable Number isNaN parseInt  -0b1          gt  false Number isNaN parseInt  -0o1          gt  false Number isNaN parseInt  -0xa          gt  false Number isNaN parseInt  Infinity          gt  true  Number isNaN parseInt  INFINITY          gt  true   Number isNaN parseInt  -Infinity          gt  true  Number isNaN parseInt  -INFINITY          gt  true   Using parseFloat     Base cases that are handled properly Number isNaN parseFloat  1          gt  false Number isNaN parseFloat  -1          gt  false Number isNaN parseFloat  1 1          gt  false Number isNaN parseFloat  -1 1          gt  false Number isNaN parseFloat  asdf          gt  true Number isNaN parseFloat undefined         gt  true Number isNaN parseFloat            gt  true Number isNaN parseFloat             gt  true Number isNaN parseFloat null         gt  true     Special notation cases that are handled properly Number isNaN parseFloat  1e1          gt  false Number isNaN parseFloat  1e-1          gt  false Number isNaN parseFloat  -1e1          gt  false Number isNaN parseFloat  -1e-1          gt  false Number isNaN parseFloat  0b1          gt  false Number isNaN parseFloat  0o1          gt  false Number isNaN parseFloat  0xa          gt  false     Edge cases that are debatable Number isNaN parseFloat  -0b1          gt  false Number isNaN parseFloat  -0o1          gt  false Number isNaN parseFloat  -0xa          gt  false Number isNaN parseFloat  Infinity          gt  false  Number isNaN parseFloat  INFINITY          gt  true   Number isNaN parseFloat  -Infinity          gt  false  Number isNaN parseFloat  -INFINITY          gt  true  Notes   Only string  empty  and uninitialized values are considered in keeping with addressing the original question   Additional edge cases exist if arrays and objects are the values being considered  Characters in binary  octal  hexadecimal  and exponential notation are not case-sensitive  ie   0xFF    0XFF    0xfF  etc  will all yield the same result in the test cases shown above   Unlike with Infinity  case-sensitive  in some cases  constants from the Number and Math objects passed as test cases in string format to any of the methods above will be determined to not be numbers  See here for an explanation of how arguments are converted to a Number and why the edge cases for null and empty strings exist

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2nd October 2020  note that many bare-bones approaches are fraught with subtle bugs  eg  whitespace  implicit partial parsing  radix  coercion of arrays etc   that many of the answers here fail to take into account  The following implementation might work for you  but note that it does not cater for number separators other than the decimal point  quot   quot   function isNumeric str      if  typeof str     quot string quot   return false    we only process strings      return  isNaN str   amp  amp     use type coercion to parse the  entirety  of the string   parseFloat  alone does not do this               isNaN parseFloat str         and ensure strings of whitespace fail     To check if a variable  including a string  is a number  check if it is not a number  This works regardless of whether the variable content is a string or number  isNaN num             returns true if the variable does NOT contain a valid number  Examples isNaN 123             false isNaN  123            false isNaN  1e10000        false  This translates to Infinity  which is a number  isNaN  foo            true isNaN  10px           true  Of course  you can negate this if you need to  For example  to implement the IsNumeric example you gave  function isNumeric num     return  isNaN num     To convert a string containing a number into a number  Only works if the string only contains numeric characters  else it returns NaN   num                  returns the numeric value of the string  or NaN                        if the string isn t purely numeric characters  Examples   12                  12   12                  12   12                  NaN    12                 0 12     12                NaN   foo                 NaN   12px                NaN  To convert a string loosely to a number Useful for converting  12px  to 12  for example  parseInt num          extracts a numeric value from the                        start of the string  or NaN   Examples parseInt  12          12 parseInt  aaa         NaN parseInt  12px        12 parseInt  foo2        NaN      These last two may be different parseInt  12a5        12       from what you expected to see    Floats Bear in mind that  unlike  num  parseInt  as the name suggests  will convert a float into an integer by chopping off everything following the decimal point  if you want to use parseInt   because of this behaviour  you re probably better off using another method instead     12 345              12 345 parseInt 12 345       12 parseInt  12 345      12  Empty strings Empty strings may be a little counter-intuitive   num converts empty strings or strings with spaces to zero  and isNaN   assumes the same                        0                       0 isNaN                 false isNaN                 false  But parseInt   does not agree  parseInt              NaN parseInt              NaN

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This appears to catch the seemingly infinite number of edge cases   function isNumber x  noStr                    - Returns true if x is either a finite number type or a string containing only a number         - If empty string supplied  fall back to explicit false         - Pass true for noStr to return false when typeof x is  string   off by default          isNumber       false         isNumber         false         isNumber  1       false         isNumber  1 2       false         isNumber         false         isNumber null      false         isNumber         false         isNumber true      false         isNumber  true       false         isNumber  false       false         isNumber  123asdf       false         isNumber  123 asdf       false         isNumber undefined      false         isNumber Number POSITIVE INFINITY      false         isNumber Number NEGATIVE INFINITY      false         isNumber  Infinity       false         isNumber  -Infinity       false         isNumber Number NaN      false         isNumber new Date  December 17  1995 03 24 00        false         isNumber 0      true         isNumber  0       true         isNumber 123      true         isNumber 123 456      true         isNumber -123 456      true         isNumber - 123456      true         isNumber  123       true         isNumber  123 456       true         isNumber   123       true         isNumber  123      true         isNumber Number MAX SAFE INTEGER      true         isNumber Number MAX VALUE      true         isNumber Number MIN VALUE      true         isNumber new Number 123       true             return            typeof x      number     x instanceof Number      noStr  amp  amp  x  amp  amp  typeof x      string   amp  amp   isNaN x     amp  amp          isFinite x           false

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Quote      isNaN num             returns true if the variable does NOT contain a valid number   is not entirely true if you need to check for leading trailing spaces - for example when a certain quantity of digits is required  and you need to get  say   1111  and not   111  or  111   for perhaps a PIN input   Better to use   var num      d    test num

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Just use isNaN    this will convert the string to a number and if get a valid number  will return false     isNaN  Alireza      return true isNaN  123      return false

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Old question  but there are several points missing in the given answers   Scientific notation    isNaN  1e 30   is true  however in most of the cases when people ask for numbers  they do not want to match things like 1e 30   Large floating numbers may behave weird  Observe  using Node js     gt  var s   Array 16   1  join  9   undefined  gt  s length 16  gt  s  9999999999999999   gt   isNaN s  true  gt  Number s  10000000000000000  gt  String Number s       s false  gt    On the other hand    gt  var s   Array 16   1  join  1   undefined  gt  String Number s       s true  gt  var s   Array 15   1  join  9   undefined  gt  String Number s       s true  gt    So  if one expects String Number s       s  then better limit your strings to 15 digits at most  after omitting leading zeros    Infinity   gt  typeof Infinity  number   gt   isNaN  Infinity   true  gt  isFinite  Infinity   false  gt    Given all that  checking that the given string is a number satisfying all of the following    non scientific notation predictable conversion to Number and back to String finite   is not such an easy task  Here is a simple version     function isNonScientificNumberString o        if   o    typeof o      string              Should not be given anything but strings        return false            return o length  lt   15  amp  amp  o indexOf  e     lt  0  amp  amp  o indexOf  E     lt  0  amp  amp   isNaN o   amp  amp  isFinite o         However  even this one is far from complete  Leading zeros are not handled here  but they do screw the length test

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Save yourself the headache of trying to find a  built-in  solution   There isn t a good answer  and the hugely upvoted answer in this thread is wrong   npm install is-number     In JavaScript  it s not always as straightforward as it should be to reliably check if a value is a number  It s common for devs to use    -  or Number   to cast a string value to a number  for example  when values are returned from user input  regex matches  parsers  etc   But there are many non-intuitive edge cases that yield unexpected results    console log           gt  0 console log           gt  0 console log              gt  0 console log typeof NaN       gt   number

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If anyone ever gets this far down  I spent some time hacking on this trying to patch moment js  https   github com moment moment   Here s something that I took away from it   function isNumeric val        var  val    val      return  val     val   1    infinity check          amp  amp    val      val    Cute coercion check          amp  amp   typeof val      object     Array object check     Handles the following cases   True     isNumeric  1    isNumeric 1e10   isNumeric 1E10   isNumeric   6e4    isNumeric  1 2222    isNumeric  -1 2222    isNumeric  -1 222200000000000000    isNumeric  1 222200000000000000    isNumeric 1   isNumeric 0   isNumeric -0   isNumeric 1010010293029   isNumeric 1 100393830000   isNumeric Math LN2   isNumeric Math PI   isNumeric 5e10     False     isNumeric NaN   isNumeric Infinity   isNumeric -Infinity   isNumeric    isNumeric undefined   isNumeric   1 2 3     isNumeric  a 1 b 2    isNumeric null   isNumeric  1    isNumeric new Date       Ironically  the one I am struggling with the most   isNumeric new Number 1     gt  false   Any suggestions welcome

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Here s a one-liner to check if sNum is a valid numeric value  it has been tested for a wide variety of inputs    isNaN  s replace   s    g            returns True if numeric value

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If you re just trying to check if a string is a whole number  no decimal places   regex is a good way to go  Other methods such as isNaN are too complicated for something so simple      function isNumeric value            return   -  d    test value                  console log isNumeric  abcd                false     console log isNumeric  123a                false     console log isNumeric  1                   true     console log isNumeric  1234567890          true     console log isNumeric  -23                 true     console log isNumeric 1234                 true     console log isNumeric  123 4               false     console log isNumeric                      false     console log isNumeric undefined            false     console log isNumeric null                 false  To only allow positive whole numbers use this      function isNumeric value            return    d    test value              console log isNumeric  123                 true     console log isNumeric  -23                 false

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I like the simplicity of this  Number isNaN Number value    The above is regular Javascript  but I m using this in conjunction with a typescript typeguard for smart type checking  This is very useful for the typescript compiler to give you correct intellisense  and no type errors  Typescript typeguards isNotNumber value  string   number   value is string       return Number isNaN Number this smartImageWidth      isNumber value  string   number   value is number       return Number isNaN Number this smartImageWidth       false     Let s say you have a property width which is number   string  You may want to do logic based on whether or not it s a string  var width  number string  width    quot 100vw quot    if  isNotNumber width             the compiler knows that width here must be a string     if  width endsWith  vw                      we have a  width  such as 100vw          else           the compiler is smart and knows width here must be number     var doubleWidth   width   2         The typeguard is smart enough to constrain the type of width within the if statement to be ONLY string  This permits the compiler to allow width endsWith      which it wouldn t allow if the type was string   number  You can call the typeguard whatever you want isNotNumber  isNumber  isString  isNotString but I think isString is kind of ambiguous and harder to read

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Here is a high-performance  2 5 10 7 iterations s  3 8GHz Haswell  version of a isNumber implementation  It works for every testcase i could find  including Symbols    var isNumber    function        var isIntegerTest      d       var isDigitArray     0   0   0   0   0   0   0   0   0   0     function hasLeading0s  s        return   typeof s      string         s length  lt  2        s 0       0          isDigitArray s 1          isIntegerTest test s          var isWhiteSpaceTest     s     return function isNumber  s        var t   typeof s      var n      if  t      number           return  s  lt   0      s  gt  0         else if  t      string           n    s        return      n  lt   0   amp  amp    n  gt  0      n      0     hasLeading0s s       n     0      s           isWhiteSpaceTest test s            else if  t      object           return     s instanceof Number       n    s     n  lt   0   amp  amp    n  gt  0               return false

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So  it will depend on the test cases that you want it to handle  function isNumeric number      return  isNaN parseFloat number    amp  amp   isNaN  number      What I was looking for was regular types of numbers in javascript  0  1   -1  1 1   -1 1   1E1   -1E1   1e1   -1e1  0 1e10  -0 1 e10   0xAF1   0o172  Math PI  Number NEGATIVE INFINITY  Number POSITIVE INFINITY And also they re representations as strings   0    1    -1    1 1    -1 1    1E1    -1E1    1e1    -1e1    0 1e10    -0 1 e10    0xAF1    0o172  I did want to leave out and not mark them as numeric                   null  undefined  NaN As of today  all other answers seemed to failed one of these test cases

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2019  Including ES3  ES6 and TypeScript Examples  Maybe this has been rehashed too many times  however I fought with this one today too and wanted to post my answer  as I didn t see any other answer that does it as simply or thoroughly   ES3  var isNumeric   function num       return  typeof num       number     typeof num       string   amp  amp  num trim            amp  amp   isNaN num         ES6  const isNumeric    num    gt   typeof num       number     typeof num       string   amp  amp  num trim            amp  amp   isNaN num     Typescript  const isNumeric    num  any    gt   typeof num       number     typeof num       string   amp  amp  num trim            amp  amp   isNaN num as number     This seems quite simple and covers all the bases I saw on the many other posts and thought up myself      Positive Cases console log 0  isNumeric 0      true   console log 1  isNumeric 1      true   console log 1234567890  isNumeric 1234567890      true   console log  1234567890   isNumeric  1234567890       true   console log  0   isNumeric  0       true   console log  1   isNumeric  1       true   console log  1 1   isNumeric  1 1       true   console log  -1   isNumeric  -1       true   console log  -1 2354   isNumeric  -1 2354       true   console log  -1234567890   isNumeric  -1234567890       true   console log -1  isNumeric -1      true   console log -32 1  isNumeric -32 1      true   console log  0x1   isNumeric  0x1       true       Valid number in hex    Negative Cases console log true  isNumeric true      false   console log false  isNumeric false      false   console log  1  1   isNumeric  1  1       false   console log  1 1   isNumeric  1 1       false   console log  -32 1 12   isNumeric  -32 1 12       false   console log   blank    isNumeric         false   console log   spaces    isNumeric            false   console log  null   isNumeric null      false   console log  undefined   isNumeric undefined      false   console log     isNumeric         false   console log  NaN   isNumeric NaN      false     You can also try your own isNumeric function and just past in these use cases and scan for  true  for all of them   Or  to see the values that each return

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I do it like this   function isString value        return value length     undefined    function isNumber value        return value NaN     undefined      Of course isString   will be tripped up here if you pass some other object that has  length  defined

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Try the isNan function      The isNaN   function determines whether a value is an illegal number  Not-a-Number        This function returns true if the value equates to NaN  Otherwise it returns false       This function is different from the Number specific Number isNaN   method        nbsp  The global isNaN   function  converts the tested value to a Number  then tests it       Number isNan   does not convert the values to a Number  and will not return true for any value that is not of the type Number

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The accepted answer for this question has quite a few flaws  as highlighted by couple of other users   This is one of the easiest  amp  proven way to approach it in javascript   function isNumeric n      return  isNaN parseFloat n    amp  amp  isFinite n       Below are some good test cases   console log isNumeric 12345678912345678912       true console log isNumeric  2                         true console log isNumeric  -32 2                     true console log isNumeric -32 2                      true console log isNumeric undefined                  false     the accepted answer fails at these tests  console log isNumeric                            false console log isNumeric null                       false console log isNumeric                            false

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2nd October 2020  note that many bare-bones approaches are fraught with subtle bugs  eg  whitespace  implicit partial parsing  radix  coercion of arrays etc   that many of the answers here fail to take into account  The following implementation might work for you  but note that it does not cater for number separators other than the decimal point  quot   quot   function isNumeric str      if  typeof str     quot string quot   return false    we only process strings      return  isNaN str   amp  amp     use type coercion to parse the  entirety  of the string   parseFloat  alone does not do this               isNaN parseFloat str         and ensure strings of whitespace fail     To check if a variable  including a string  is a number  check if it is not a number  This works regardless of whether the variable content is a string or number  isNaN num             returns true if the variable does NOT contain a valid number  Examples isNaN 123             false isNaN  123            false isNaN  1e10000        false  This translates to Infinity  which is a number  isNaN  foo            true isNaN  10px           true  Of course  you can negate this if you need to  For example  to implement the IsNumeric example you gave  function isNumeric num     return  isNaN num     To convert a string containing a number into a number  Only works if the string only contains numeric characters  else it returns NaN   num                  returns the numeric value of the string  or NaN                        if the string isn t purely numeric characters  Examples   12                  12   12                  12   12                  NaN    12                 0 12     12                NaN   foo                 NaN   12px                NaN  To convert a string loosely to a number Useful for converting  12px  to 12  for example  parseInt num          extracts a numeric value from the                        start of the string  or NaN   Examples parseInt  12          12 parseInt  aaa         NaN parseInt  12px        12 parseInt  foo2        NaN      These last two may be different parseInt  12a5        12       from what you expected to see    Floats Bear in mind that  unlike  num  parseInt  as the name suggests  will convert a float into an integer by chopping off everything following the decimal point  if you want to use parseInt   because of this behaviour  you re probably better off using another method instead     12 345              12 345 parseInt 12 345       12 parseInt  12 345      12  Empty strings Empty strings may be a little counter-intuitive   num converts empty strings or strings with spaces to zero  and isNaN   assumes the same                        0                       0 isNaN                 false isNaN                 false  But parseInt   does not agree  parseInt              NaN parseInt              NaN

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I used this function as a form validation tool  and I didn t want users to be able to write exponential function  so I came up with this function   lt script gt       function isNumber value  acceptScientificNotation             if true     acceptScientificNotation               return   - 0 1  d     d      test value                      if  true     Array isArray value                 return false                    return  isNaN parseInt value  10                console log isNumber  quot  quot                     false     console log isNumber false                 false     console log isNumber true                  false     console log isNumber  quot 0 quot                    true     console log isNumber  quot 0 1 quot                  true     console log isNumber  quot 12 quot                   true     console log isNumber  quot -12 quot                  true     console log isNumber -45                   true     console log isNumber  jo   quot pi quot              false     console log isNumber                       false     console log isNumber  78  79               false     console log isNumber NaN                   false     console log isNumber Infinity              false     console log isNumber undefined             false     console log isNumber  quot 0 1 quot                  false        console log isNumber  quot 1e-1 quot                 false     console log isNumber  quot 1e-1 quot   true          true  lt  script gt

[javascript] (Built-in) way in JavaScript to check if a string is a valid number

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