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Truong Phan
Truong Phan

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Pair of concepts may confuse you in Javascript - Number.isNaN() and isNaN()

With a long history of development (and chaos), JS has some messy legacy that can't be removed to keep the consistency but only improved by new features/functions. This leads to confusion for developers. This series is written as notes for myself and others to comprehend these concepts and avoid bugs in development.

What is NaN?

NaN is shorthand for Not A Number, specified in The IEEE Standard for Floating-Point Arithmetic (IEEE 754-2008) for floating-point arithmetic established in 1985. In Javascript context, it is a "property of the global object. In other words, it is a variable in global scope.". It has below characteristics:

  • It is considered a Number type
  • Equalivent to Number.NaN
  • NaN is the only value in JavaScript which is not equal to itself.
  • It's falsy

console.log(NaN === NaN) // false
console.log(NaN == NaN) // false

console.log(NaN !== NaN) // true
console.log(NaN != NaN) // true
console.log(typeof(NaN)) // number
a = NaN;
a ? true : false //false
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As you can see NaN even can not be compared to itself, so how we can detect if a variable is a NaN, before ES6 we can use the function isNaN(), yet considered the following examples.

isNaN(NaN); // true
isNaN('NaN');   // true
isNaN(undefined); // true
isNaN({}); // true
isNaN(''); // true
isNaN(''); // false
isNaN('12abcd') // true
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To understand this behavior, we need to understand how it works properly.
According to MDN: "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"

Then, many people argue that its behavior for non-numeric arguments has been confusing and may cause unexpected results. As the result, the new function was introduced in ECMAScript 2015 (ES6) to solve this problem.


It is a static function from the primitive wrapper object - Number. The most important feature of the function is that it doesn't force converting the argument to a number. Because NaN is the only value in JavaScript that is not equal to itself, Number.isNaN() has been claimed it is necessary.

Number.isNaN(NaN);        // true
Number.isNaN(Number.NaN); // true
Number.isNaN(0 / 0);      // true

// e.g. these would have been true with global isNaN()
Number.isNaN('NaN');      // false
Number.isNaN(undefined);  // false
Number.isNaN({});         // false
Number.isNaN('');   // false
Number.isNaN(''); // false
Number.isNaN('12abcd') // false

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From my personal point of view, isNaN() may not be a bug as many people thought but it could be considered when you want to focus on the detection of value. The thing is that we need to comprehend its mechanism that it will try to convert arguments to Number. For reliability, we should implement Number.isNaN() when we want to make sure its argument is Number for the comparison.


As the article has been discussed actively on Reddit, and there is a discussion about Polyfill, so here is a quick and short Polyfill for Number.isNaN() in case you want to support ES5 and previous versions.

if (!Number.isNaN) {
  Number.isNaN = function(n) {
    return n !== n;
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Originally published at The Techika

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