How do you check that a number is NaN in JavaScript

Question

I   ve only been trying it in Firefox   s JavaScript console  but neither of the following statements return true   parseFloat  geoff      NaN   parseFloat  geoff      Number NaN

User · Answer

As far as a value of type Number is to be tested whether it is a NaN or not, the global function isNaN will do the work

isNaN(any-Number);

For a generic approach which works for all the types in JS, we can use any of the following:

For ECMAScript-5 Users:

#1
if(x !== x) {
    console.info('x is NaN.');
}
else {
    console.info('x is NOT a NaN.');
}

For people using ECMAScript-6:

#2
Number.isNaN(x);

And For consistency purpose across ECMAScript 5 & 6 both, we can also use this polyfill for Number.isNan

#3
//Polyfill from MDN
Number.isNaN = Number.isNaN || function(value) {
    return typeof value === "number" && isNaN(value);
}
// Or
Number.isNaN = Number.isNaN || function(value) {     
    return value !== value;
}

please check This Answer for more details.

User · Answer

So I see several responses to this   But I just use   function isNaN x        return x    x  amp  amp  typeof x     number

User · Answer

The rule is   NaN    NaN   The problem of isNaN   function is that it may return unexpected result in some cases   isNaN  Hello          true isNaN  2005 12 12     true isNaN undefined       true isNaN  NaN            true isNaN NaN             true isNaN 0   0           true   A better way to check if the value is really NaN is   function is nan value        return value    value    is nan parseFloat  geoff

User · Answer

marksyzm s answer works well  but it does not return false for Infinity as Infinity is techinicly not a number   i came up with a isNumber function that will check if it is a number    x000D   x000D  function isNumber i    x000D      return  isNaN i  amp  amp  i     true   Number i    parseFloat i    amp  amp   Number POSITIVE INFINITY  Number NEGATIVE INFINITY  indexOf i      -1  x000D    x000D   x000D  console log isNumber Infinity    x000D  console log isNumber  asdf     x000D  console log isNumber 1 4    x000D  console log isNumber NaN    x000D  console log isNumber Number MAX VALUE    x000D  console log isNumber  1 68     x000D   x000D   x000D    UPDATE  i noticed that this code fails for some parameters  so i made it better    x000D   x000D  function isNumber i     function for checking if parameter is number x000D  if  arguments length    x000D  throw new SyntaxError  not enough arguments     x000D     else if arguments length  gt  1    x000D  throw new SyntaxError  too many arguments     x000D     else if  Number NEGATIVE INFINITY  Number POSITIVE INFINITY  indexOf i      -1    x000D  throw new RangeError  number cannot be  xB1infinity     x000D     else if typeof i      object   amp  amp    i instanceof RegExp   amp  amp    i instanceof Number   amp  amp    i     null     x000D  throw new TypeError  parameter cannot be object array     x000D     else if i instanceof RegExp    x000D  throw new TypeError  parameter cannot be RegExp     x000D     else if i    null    i     undefined    x000D  throw new ReferenceError  parameter is null or undefined     x000D     else   x000D  return  isNaN i  amp  amp  i     true   Number i    parseFloat i    amp  amp   i     i   x000D     x000D    x000D  console log isNumber Infinity    x000D  console log isNumber this    x000D  console log isNumber    ig    x000D  console log isNumber null    x000D   x000D   x000D

User · Answer

Simple Solution    REALLY super simple  Here  Have this method   function isReallyNaN a    return a     a       Use as simple as   if   isReallyNaN value     return doingStuff         See performance test here using this func vs selected answer      Also  See below 1st example for a couple alternate implementations      Example    x000D   x000D  function isReallyNaN a    return a     a     x000D   x000D  var example     x000D       NaN   NaN  x000D       an empty Objet       x000D       a parse to NaN   parseFloat   5 32    x000D       a non-empty Objet     a  1  b  2    x000D       an empty Array       x000D       a semi-passed parse   parseInt  5a5    x000D       a non-empty Array      a    b    c     x000D       Math to NaN   Math log -1   x000D       an undefined object   undefined x000D      x000D   x000D  for  x in example    x000D      var answer   isReallyNaN example x    x000D          strAnswer   answer toString    x000D         table   append     lt tr   gt       class   strAnswer    append     lt th   gt      x000D          html  x x000D              lt td   gt      x000D          html  strAnswer x000D           x000D     x000D  table   border-collapse  collapse    x000D  th  td   border  1px solid  padding  2px 5px    x000D   true   color  red    x000D   false   color  green    x000D   lt script src  https   ajax googleapis com ajax libs jquery 1 9 1 jquery min js  gt  lt  script gt  x000D   lt table gt  lt  table gt  x000D   x000D   x000D    There are a couple alternate paths you take for implementaion  if you don t want to use an alternately named method  and would like to ensure it s more globally available  Warning These solutions involve altering native objects  and may not be your best solution  Always use caution and be aware that other Libraries you might use may depend on native code or similar alterations   Alternate Implementation 1  Replace Native isNaN method       Extremely simple  Just simply write the method  window isNaN   function a    return a    a      Alternate Implementation 2  Append to Number Object Suggested as it is also a poly-fill for ECMA 5 to 6  Number  isNaN       Number isNaN   function a    return a     a         Use as simple as Number isNaN NaN      Alternate solution test if empty  A simple window method I wrote that test if object is Empty  It s a little different in that it doesn t give if item is  exactly  NaN  but I figured I d throw this up as it may also be useful when looking for empty items       isEmpty varried      Simple method for testing if item is  empty         function        function isEmpty a    return   a    0  gt   a       object     typeof a  amp  amp           null          test JSON stringify a           window hasOwnProperty  empty     window empty isEmpty           Example    x000D   x000D    function     x000D     function isEmpty a    return  a    void 0     a    a     a    0  gt   a     object     typeof a  amp  amp           null          test JSON stringify a       x000D     window hasOwnProperty  empty     window empty isEmpty   x000D        x000D   x000D  var example     x000D       NaN   NaN  x000D       an empty Objet       x000D       a parse to NaN   parseFloat   5 32    x000D       a non-empty Objet     a  1  b  2    x000D       an empty Array   new Array    x000D       an empty Array w  9 len   new Array 9   x000D       a semi-passed parse   parseInt  5a5    x000D       a non-empty Array      a    b    c     x000D       Math to NaN   Math log -1   x000D       an undefined object   undefined x000D      x000D   x000D  for  x in example    x000D   var answer   empty example x    x000D    strAnswer   answer toString    x000D       t1   append  x000D        lt tr   gt       class   strAnswer    append  x000D         lt th   gt      html  x     x000D         lt td   gt      html  strAnswer toUpperCase      x000D      x000D     x000D     x000D   x000D   x000D  function isReallyNaN a    return a     a     x000D  for x in example  var answer isReallyNaN example x   strAnswer answer toString       t2   append     lt tr   gt     class  strAnswer   append     lt th   gt    html x       lt td   gt    html strAnswer toUpperCase         x000D  table   border-collapse  collapse  float  left    x000D  th  td   border  1px solid  padding  2px 5px    x000D   true   color  red    x000D   false   color  green    x000D   lt script src  https   ajax googleapis com ajax libs jquery 1 9 1 jquery min js  gt  lt  script gt  x000D   lt table id  t1  gt  lt thead gt  lt tr gt  lt th colspan  2  gt isEmpty   lt  th gt  lt  tr gt  lt  thead gt  lt thead gt  lt tr gt  lt th gt Value Type lt  th gt  lt th gt Bool Return lt  th gt  lt  tr gt  lt  thead gt  lt  table gt  x000D   lt table id  t2  gt  lt thead gt  lt tr gt  lt th colspan  2  gt isReallyNaN   lt  th gt  lt  tr gt  lt  thead gt  lt thead gt  lt tr gt  lt th gt Value Type lt  th gt  lt th gt Bool Return lt  th gt  lt  tr gt  lt  thead gt  lt  table gt  x000D   x000D   x000D      Extremely Deep Check If Is Empty  This last one goes a bit deep  even checking if an Object is full of blank Objects  I m sure it has room for improvement and possible pits  but so far  it appears to catch most everything     x000D   x000D  function isEmpty a    x000D   if   a    0  gt   a  return  0  x000D   if   object     typeof a    x000D    var b   JSON stringify a  replace           0  0   false null         null            g      replace                 g       x000D    if                  test b    return  0  x000D    else if  a instanceof Array     x000D     b   b replace   0  0   false null         null            g       x000D     if                  test b    return  0  x000D      x000D     x000D   return false  x000D    x000D  window hasOwnProperty  empty     window empty isEmpty   x000D   x000D  var example     x000D       NaN   NaN  x000D       an empty Objet       x000D       a parse to NaN   parseFloat   5 32    x000D       a non-empty Objet     a  1  b  2    x000D       an empty Array   new Array    x000D       an empty Array w  9 len   new Array 9   x000D       a semi-passed parse   parseInt  5a5    x000D       a non-empty Array      a    b    c     x000D       Math to NaN   Math log -1   x000D       an undefined object   undefined  x000D       Object Full of Empty Items     1      2      3      4  false  5 new Array 3   6  NaN  7  null  8  void 0  9  0  10   0   11    6  NaN  7  null  8  void 0      x000D       Array Full of Empty Items             false  null null null  null null null 0  0    6  null  7  null   x000D      x000D   x000D  for  x in example    x000D   var answer   empty example x    x000D    strAnswer   answer toString    x000D       t1   append  x000D        lt tr   gt       class   strAnswer    append  x000D         lt th   gt      html  x     x000D         lt td   gt      html  strAnswer toUpperCase      x000D      x000D     x000D     x000D   x000D   x000D  function isReallyNaN a    return a     a     x000D  for x in example  var answer isReallyNaN example x   strAnswer answer toString       t2   append     lt tr   gt     class  strAnswer   append     lt th   gt    html x       lt td   gt    html strAnswer toUpperCase         x000D  table   border-collapse  collapse  float  left    x000D  th  td   border  1px solid  padding  2px 5px    x000D   true   color  red    x000D   false   color  green    x000D   lt script src  https   ajax googleapis com ajax libs jquery 1 9 1 jquery min js  gt  lt  script gt  x000D   lt table id  t1  gt  lt thead gt  lt tr gt  lt th colspan  2  gt isEmpty   lt  th gt  lt  tr gt  lt  thead gt  lt thead gt  lt tr gt  lt th gt Value Type lt  th gt  lt th gt Bool Return lt  th gt  lt  tr gt  lt  thead gt  lt  table gt  x000D   lt table id  t2  gt  lt thead gt  lt tr gt  lt th colspan  2  gt isReallyNaN   lt  th gt  lt  tr gt  lt  thead gt  lt thead gt  lt tr gt  lt th gt Value Type lt  th gt  lt th gt Bool Return lt  th gt  lt  tr gt  lt  thead gt  lt  table gt  x000D   x000D   x000D

User · Answer

You should use the global isNaN value  function call  because    It is supported cross-browser See isNaN for documentation   Examples    isNaN  geoff       true  isNaN  3       false   I hope this will help you

User · Answer

To fix the issue where  1 2geoff  becomes parsed  just use the Number   parser instead   So rather than this   parseFloat  1 2geoff         gt  1 2 isNaN parseFloat  1 2geoff          gt  false isNaN parseFloat   2geoff          gt  false isNaN parseFloat  geoff          gt  true   Do this   Number  1 2geoff         gt  NaN isNaN Number  1 2geoff          gt  true isNaN Number   2geoff          gt  true isNaN Number  geoff          gt  true   EDIT  I just noticed another issue from this though    false values  and true as a real boolean  passed into Number   return as 0  In which case    parseFloat works every time instead  So fall back to that   function definitelyNaN  val        return isNaN val  amp  amp  val     true   Number val    parseFloat val        And that covers seemingly everything  I benchmarked it at 90  slower than lodash s   isNaN but then that one doesn t cover all the NaN s   http   jsperf com own-isnan-vs-underscore-lodash-isnan  Just to be clear  mine takes care of the human literal interpretation of something that is  Not a Number  and lodash s takes care of the computer literal interpretation of checking if something is  NaN

User · Answer

NaN is a special value that can t be tested like that  An interesting thing I just wanted to share is this  var nanValue   NaN  if nanValue     nanValue     Returns true      alert  nanValue is NaN      This returns true only for NaN values and Is a safe way of testing  Should definitely be wrapped in a function or atleast commented  because It doesnt make much sense obviously to test if the same variable is not equal to each other  hehe

User · Answer

Is  NaN    0           I don t Know    function IsNotNumber  i        if  i  gt   0    return false        if  i  lt   0    return false        return true      Conditions only execute if TRUE   Not on FALSE   Not on  I Don t Know

User · Answer

alert  1234567890   indexOf String fromCharCode mycharacter   gt -1     This is not elegant  but after trying isNAN   I arrived at this solution which is another alternative  In this example I also allowed     because I am masking for float  You could also reverse this to make sure no numbers are used      1234567890  indexOf String fromCharCode mycharacter    -1    This is a single character evaluation but you could also loop through a string to check for any numbers

User · Answer

I use underscore s isNaN function because in JavaScript   isNaN undefined   - gt  true   At the least  be aware of that gotcha

User · Answer

Found another way  just for fun   function IsActuallyNaN obj      return  obj  includes NaN

User · Answer

If your environment supports ECMAScript 2015  then you might want to use Number isNaN to make sure that the value is really NaN   The problem with isNaN is  if you use that with non-numeric data there are few confusing rules  as per MDN  are applied  For example   isNaN NaN            true isNaN undefined      true isNaN                true   So  in ECMA Script 2015 supported environments  you might want to use   Number isNaN parseFloat  geoff

User · Answer

I just want to share another alternative  it s not necessarily better than others here  but I think it s worth looking at   function customIsNaN x    return  typeof x     number   amp  amp  x    0  amp  amp   x       The logic behind this is that every number except 0 and NaN are cast to true   I ve done a quick test  and it performs as good as Number isNaN and as checking against itself for false  All three perform better than isNan  The results  customIsNaN NaN                 true customIsNaN 0 0                 true customIsNaN  new Date           true  customIsNaN 0               false customIsNaN false           false customIsNaN null            false customIsNaN undefined       false customIsNaN                 false customIsNaN                 false   May become useful if you want to avoid the broken isNaN function

User · Answer

Use this code   isNaN  geoff      See isNaN   docs on MDN   alert   isNaN  abcd         alerts true alert   isNaN  2 0         alerts false alert   isNaN 2 0        alerts false

User · Answer

Another solution is mentioned in MDN s parseFloat page  It provides a filter function to do strict parsing  var filterFloat   function  value        if     -       0-9      0-9     Infinity           test value         return Number value     return NaN      console log filterFloat  421                      421 console log filterFloat  -421                     -421 console log filterFloat   421                     421 console log filterFloat  Infinity                 Infinity console log filterFloat  1 61803398875            1 61803398875 console log filterFloat  421e 0                   NaN console log filterFloat  421hop                   NaN console log filterFloat  hop1 61803398875         NaN   And then you can use isNaN to check if it is NaN

User · Answer

I just came across this technique in the book Effective JavaScript that is pretty simple   Since NaN is the only JavaScript value that is treated as unequal to itself  you can always test if a value is NaN by checking it for equality to itself   var a   NaN  a     a     true   var b    foo   b     b     false   var c   undefined   c     c     false  var d       d     d     false  var e     valueOf   foo      e     e     false   Didn t realize this until  allsyed commented  but this is in the ECMA spec  https   tc39 github io ecma262  sec-isnan-number

User · Answer

The exact way to check is     takes care of boolen  undefined and empty  isNaN x     typeof x      boolean     typeof x      undefined     x         is really a nan     is a number

User · Answer

x000D   x000D  function isNotANumber n    x000D    if  typeof n      number     x000D      return true  x000D       x000D    return n     n  x000D    x000D   x000D   x000D

User · Answer

While  chiborg  s answer IS correct  there is more to it that should be noted   parseFloat  1 2geoff         gt  1 2 isNaN parseFloat  1 2geoff          gt  false isNaN parseFloat   2geoff          gt  false isNaN parseFloat  geoff          gt  true   Point being  if you re using this method for validation of input  the result will be rather liberal   So  yes you can use parseFloat string   or in the case of full numbers parseInt string  radix   and then subsequently wrap that with isNaN    but be aware of the gotcha with numbers intertwined with additional non-numeric characters

User · Answer

Try this code   isNaN parseFloat  geoff      For checking whether any value is NaN  instead of just numbers  see here  How do you test for NaN in Javascript

User · Answer

According to IEEE 754  all relationships involving NaN evaluate as false except      Thus  for example   A    B    false and  A  lt   B    false if A or B or both is are NaN

User · Answer

As of ES6  Object is     is a new utility that can be used to test two values for absolute equality   var a   3    bar   Object is a  NaN      true

User · Answer

It seems that isNaN   is not supported in Node js out of the box  I worked around with    var value   1  if  parseFloat stringValue          NaN   value   parseFloat stringValue

User · Answer

I wrote this answer to another question on StackOverflow where another checks when NaN    null but then it was marked as duplicate so I don t want to waste my job  Look at Mozilla Developer Network about NaN   Short answer Just use distance    0 when you want to be sure you value is a proper number or isNaN   to check it  Long answer The NaN  Not-a-Number  is a weirdo Global Object in javascript frequently returned when some mathematical operation failed  You wanted to check if NaN    null which results false  Hovewer even NaN    NaN results with false  A Simple way to find out if variable is NaN is an global function isNaN    Another is x     x which is only true when x is NaN   thanks for remind to  raphael-schweikert  But why the short answer worked  Let s find out  When you call NaN    false the result is false  same with NaN    true  Somewhere in specifications JavaScript has an record with always false values  which includes   NaN - Not-a-Number  quot  quot  - empty string false - a boolean false null - null object undefined - undefined variables 0 - numerical 0  including  0 and -0

User · Answer

NaN in JavaScript stands for  Not A Number   although its type is actually number   typeof NaN      number    To check if a variable is of value NaN  we cannot simply use function isNaN    because isNaN   has the following issue  see below   var myVar    A   isNaN myVar     true  although  A  is not really of value NaN   What really happens here is that myVar is implicitly coerced to a number   var myVar    A   isNaN Number myVar      true  Number myVar  is NaN here in fact   It actually makes sense  because  A  is actually not a number  But what we really want to check is if myVar is exactly of value NaN   So isNaN   cannot help  Then what should we do instead   In the light that NaN is the only JavaScript value that is treated unequal to itself  so we can check for its equality to itself using      var myVar     undefined myVar     myVar    false  var myVar    A   myVar     myVar    false  var myVar   NaN myVar     myVar    true   So to conclude  if it is true that a variable     itself  then this variable is exactly of value NaN   function isOfValueNaN v        return v     v     var myVar    A   isNaN myVar      true isOfValueNaN myVar      false

User · Answer

Maybe also this   function isNaNCustom value       return value toString        NaN   amp  amp              typeof value      string   amp  amp              typeof value      number

User · Answer

I ve created this little function that works like a charm  Instead of checking for NaN which seems to be counter intuitive  you check for a number  I m pretty sure I am not the first to do it this way  but I thought i d share   function isNum val       var absVal   Math abs val       var retval   false      if  absVal-absVal     0           retval   true            return retval

User · Answer

Simply convert the result to String and compare with  NaN    var val   Number  test    if String val       NaN        console log  true

User · Answer

NaN     NaN            false Number NaN     NaN     false isNaN NaN              true isNaN Number NaN       true      Equality operator     and      cannot be used to test a value against NaN    Look at Mozilla Documentation The global NaN property is a value representing Not-A-Numbe  The best way is using  isNaN    which is buit-in function  to check NaN  All browsers supports the way

[javascript] How do you check that a number is NaN in JavaScript?

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