How to Check if Number is NaN in JavaScript

Check if Number is NaN in JavaScript #

To check if a number is NaN, call the Number.isNaN() method, passing it the number as a parameter. The Number.isNaN method returns true if the passed in value is NaN and has a type of number, otherwise it returns false.

Copied!
const num = 0 / 'test';
console.log(num); // 👉️ NaN

if (Number.isNaN(num)) {
  // 👉️ this runs only if NaN and type of number
  console.log('Number is NaN');
}

We use the Number.isNaN method to check if a value has a type of number and is NaN.

You should never try the equality operators when checking for NaN because as confusing as it may seem, NaN is not equal to NaN in Javascript.

Copied!
// 👇️ don't do this
console.log(Number.NaN === Number.NaN); // 👉️ false

The Number.isNaN only returns true if the following conditions are met:

• the value is of type number
• the value is NaN

All the following examples return false.

Copied!
console.log(Number.isNaN('string')); // 👉️ false
console.log(Number.isNaN(undefined)); // 👉️ false
console.log(Number.isNaN(null)); // 👉️ false
console.log(Number.isNaN([])); // 👉️ false
console.log(Number.isNaN({})); // 👉️ false

If you need to support Internet Explorer add the following polyfill to your script, before you try to use the Number.isNaN method.

Copied!
Number.isNaN = Number.isNaN || function isNaN(input) {
    return typeof input === 'number' && input !== input;
}

This code is quite intuitive, we first check if the Number.isNaN method is supported by the browser.

If it isn't supported by the browser we implement it by attaching a function to the isNaN property of the Number object.

Our function checks if the type of the provided value is of type number and it's not equal to itself.

Since NaN is the only value in JavaScript that is not equal to itself this is sufficient to determine whether the user input is NaN.

Hopefully they don't come up with other values that are not equal to themselves, that would break our polyfill.

An alternative approach is to use the older isNaN method, which is supported by Internet Explorer.

If the argument provided to the isNaN method is not of type number, the method coerces the value to a number, before checking if it's NaN.

This is different from the Number.isNaN method, which doesn't coerce the passed in value.

Here are some examples of using the strange isNaN method:

Copied!
console.log(isNaN(0 / 0)); // 👉️ true
console.log(isNaN('string')); // 👉️ true
console.log(isNaN(undefined)); // 👉️ true
console.log(isNaN({})); // 👉️ true
console.log(isNaN(null)); // 👉️ false
console.log(isNaN([])); // 👉️ false

Had we used the newer Number.isNaN method with these values, the only true response would come from calling the method with 0 / 0.

It is generally advised against using methods that do more than they need to and might surprise you and cause difficult to track bugs in your application.