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The isNaN() function determines whether a value is NaN or not. Note: coercion inside the isNaN function has interesting rules; you may alternatively want to use Number.isNaN(), as defined in ECMAScript 2015, or you can use typeof to determine if the value is Not-A-Number.




The value to be tested.

Return value

true if the given value is NaN; otherwise, false.


The necessity of an isNaN function

Unlike all other possible values in JavaScript, it is not possible to rely on the equality operators (== and ===) to determine whether a value is NaN or not, because both NaN == NaN and NaN === NaN evaluate to false. Hence, the necessity of an isNaN function.

Origin of NaN values

NaN values are generated when arithmetic operations result in undefined or unrepresentable values. Such values do not necessarily represent overflow conditions. A NaN also results from attempted coercion to numeric values of non-numeric values for which no primitive numeric value is available.

For example, dividing zero by zero results in a NaN — but dividing other numbers by zero does not.

Confusing special-case behavior

Since the very earliest versions of the isNaN function specification, its behavior for non-numeric arguments has been confusing. When the argument to the isNaN function is not of type Number, the value is first coerced to a Number. The resulting value is then tested to determine whether it is NaN. Thus for non-numbers that when coerced to numeric type result in a valid non-NaN numeric value (notably the empty string and boolean primitives, which when coerced give numeric values zero or one), the "false" returned value may be unexpected; the empty string, for example, is surely "not a number." The confusion stems from the fact that the term, "not a number", has a specific meaning for numbers represented as IEEE-754 floating-point values. The function should be interpreted as answering the question, "is this value, when coerced to a numeric value, an IEEE-754 'Not A Number' value?"

The latest version of ECMAScript (ES2015) contains the Number.isNaN() function. Number.isNaN(x) will be a reliable way to test whether x is NaN or not. Even with Number.isNaN, however, the meaning of NaN remains the precise numeric meaning, and not simply, "not a number". Alternatively, in absense of Number.isNaN, the expression (x != x) is a more reliable way to test whether variable x is NaN or not, as the result is not subject to the false positives that make isNaN unreliable.

You could think of isNaN as:

var isNaN = function(value) {
    return Number.isNaN(Number(value));


isNaN(NaN);       // true
isNaN(undefined); // true
isNaN({});        // true

isNaN(true);      // false
isNaN(null);      // false
isNaN(37);        // false

// strings
isNaN('37');      // false: "37" is converted to the number 37 which is not NaN
isNaN('37.37');   // false: "37.37" is converted to the number 37.37 which is not NaN
isNaN('123ABC');  // true:  parseInt("123ABC") is 123 but Number("123ABC") is NaN
isNaN('');        // false: the empty string is converted to 0 which is not NaN
isNaN(' ');       // false: a string with spaces is converted to 0 which is not NaN

// dates
isNaN(new Date());                // false
isNaN(new Date().toString());     // true

// This is a false positive and the reason why isNaN is not entirely reliable
isNaN('blabla');   // true: "blabla" is converted to a number. 
                   // Parsing this as a number fails and returns NaN

Useful special-case behavior

There is a more usage oriented way to think of isNaN(): If isNaN(x) returns false, you can use x in an arithmetic expression not making the expression return NaN. If it returns true, x will make every arithmetic expression return NaN. This means that in JavaScript, isNaN(x) == true is equivalent to x - 0 returning NaN (though in JavaScript x - 0 == NaN always returns false, so you can't test for it). Actually, isNaN(x), isNaN(x - 0), isNaN(Number(x)), Number.isNaN(x - 0), and Number.isNaN(Number(x)) always return the same and in JavaScript isNaN(x) is just the shortest possible form to express each of these terms.

You can use this, for example, to test whether an argument to a function is arithmetically processable (usable "like" a number), or if it's not and you have to provide a default value or something else. This way you can have a function that makes use of the full versatility JavaScript provides by implicitly converting values depending on context.


function increment(x) {
  if (isNaN(x)) x = 0;
  return x + 1;

// The same effect with Number.isNaN():
function increment(x) {
  if (Number.isNaN(Number(x))) x = 0;
  return x + 1;

// In the following cases for the function's argument x,
// isNaN(x) is always false, although x is indeed not a
// number, but can be used as such in arithmetical
// expressions
increment('');            // 1: "" is converted to 0
increment(new String());  // 1: String object representing an empty string is converted to 0
increment([]);            // 1: [] is converted to 0
increment(new Array());   // 1: Array object representing an empty array is converted to 0
increment('0');           // 1: "0" is converted to 0
increment('1');           // 2: "1" is converted to 1
increment('0.1');         // 1.1: "0.1" is converted to 0.1
increment('Infinity');    // Infinity: "Infinity" is converted to Infinity
increment(null);          // 1: null is converted to 0
increment(false);         // 1: false is converted to 0
increment(true);          // 2: true is converted to 1
increment(new Date());    // returns current date/time in milliseconds plus 1

// In the following cases for the function's argument x,
// isNaN(x) is always false and x is indeed a number
increment(-1);            // 0
increment(-0.1);          // 0.9
increment(0);             // 1
increment(1);             // 2
increment(2);             // 3
// ... and so on ...
increment(Infinity);      // Infinity

// In the following cases for the function's argument x,
// isNaN(x) is always true and x is really not a number,
// thus the function replaces it by 0 and returns 1
increment(String);            // 1
increment(Array);             // 1
increment('blabla');          // 1
increment('-blabla');         // 1
increment(0 / 0);               // 1
increment('0 / 0');             // 1
increment(Infinity / Infinity); // 1
increment(NaN);               // 1
increment(undefined);         // 1
increment();                  // 1

// isNaN(x) is always the same as isNaN(Number(x)),
// but the presence of x is mandatory here!
isNaN(x) == isNaN(Number(x)); // true for every value of x, including x == undefined,
                              // because isNaN(undefined) == true and Number(undefined) returns NaN,
                              // but ...
isNaN() == isNaN(Number());   // false, because isNaN() == true and Number() == 0


Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition.
ECMAScript 5.1 (ECMA-262)
The definition of 'isNaN' in that specification.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'isNaN' in that specification.
ECMAScript Latest Draft (ECMA-262)
The definition of 'isNaN' in that specification.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

See also