Check if a Number is NaN in JavaScript

Check if a Number is NaN in JavaScript #

Use the Number.isNaN() method to check if a number is NaN, e.g. if (Number.isNaN(num)) {}. The Number.isNaN() method will return true if the provided value is NaN and has a type of number.

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

You shouldn't use the equality operators when checking for NaN, because NaN is not equal to NaN in Javascript.

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

The Number.isNaN() method 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.

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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

Here is a polyfill that demonstrates how the Number.isNaN method is implemented.

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

We first check if the Number.isNaN method is supported by the browser.

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

The function checks if the type of the provided value is of type number and is not equal to itself.

An alternative approach is to use the older isNaN method.

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

Here are some examples of using the isNaN method:

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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

The Number.isNaN method would only return true for Number.isNaN(0 / 0). All other examples would return false.

The older isNaN method is a bit more unintuitive than Number.isNaN because it tries to coerce non-numbers to a number before checking if the value is NaN.