Check if a value is a number in JavaScript

JavaScript provides us with a few ways to check if a value is a number. We'll look over three of those ways and show the best way to check for a numeric value in JavaScript.

isNaN()

The isNaN() function checks if the passed value is not a number or can't be converted into a Number. If isNaN() returns false, the value is a number and can't be converted into a Number. Let's see a few examples.

const value = 100

isNaN(value) // false
isNaN(5.0) // false
isNaN('test') // true
isNaN([]) // false
isNaN({}) // true
isNaN(NaN) // true
isNaN(Number.NaN) // true
isNaN(0/0) // true

isNaN also returns true for cases where the value is not actually the value NaN. For example, the string 'test', is not a number and cannot be converted into a number, but it is not NaN. Yet, isNaN('test') returns true.

When isNaN() receives anything but a number, it coerces the value to a number. As a result, isNaN() can produce unexpected results. It's better to use Number.isNaN() instead of isNaN() to avoid bugs.

Number.isNaN()

The Number.isNaN() method is a more reliable way to check if a value is a number. Number.isNaN() will only check if the value passed to it is NaN and its type is Number.

Let's compare Number.isNaN() with isNaN().

Number.isNaN("foo") // false
isNaN("foo") // true

Remember that isNaN() tries to coerce "foo" into a number and can give surprising results. In this case, it gives us the wrong result, telling us that the string "foo" is the value NaN, but it's actually not.

Number.isNaN() gives us the correct answer. The string "foo" is not the value NaN and is not of type number, so it returns false. This simple example shows that Number.isNaN() is the more robust method of the two, and it should be used instead of isNaN().

Let's see a few examples of Number.isNaN().

const value = 100

Number.isNaN(value) // false
Number.isNaN(5.0) // false
Number.isNaN('test') // false
Number.isNaN([]) // false
Number.isNaN({}) // false
Number.isNaN(NaN) // true
Number.isNaN(Number.NaN) // true
Number.isNaN(0/0) // true

The values NaN, Number.NaN and 0/0 return true while all others return false. Division by zero results in NaN.

typeof

Another way to check if a value is a number in JavaScript is to use the typeof operator. When used on a numeric value, it will return a "number" string.

typeof 100 // "number"
typeof NaN // "number"
typeof {} // "object"
typeof [] // "object"
typeof null // "object"

Notice that the value NaN, meaning Not-A-Number, is actually of type number.

We can write a conditional statement to check if a value is a number using the typeof operator.

const value = 100;

if (typeof value === 'number') {
  console.log('A number');
} else {
  console.log('NOT a number');
}

Notice above that typeof NaN is actually of type number. This means that using typeof value === 'number' in a conditional expression can lead to misleading results. If value is set to NaN, the conditional check would pass as true. It would be a false positive.

We probably don't want something that is actually Not-A-Number (NaN) to make a numeric check pass. Therefore, we should combine the Number.isNaN() method with the typeof operator to make sure that NaN values don't end up causing unexpected results.

const value = 100;

if (typeof value === 'number' && !Number.isNaN(value)) {
  console.log('A number');
} else {
  console.log('NOT a number');
}

The ! operator (the logical NOT operator) was used on the Number.isNaN() method because we only want to enter the if statement when the value is NOT NaN.

Conclusion

• Avoid isNaN() for checking if a value is a number.
• Use Number.isNaN() instead of isNaN().
• If you are using the typeof operator to check if a value is a number, combine the typeof operator with Number.isNaN() for best results.