How to check if a variable is a number in JavaScript

To check if a variable is a number in JavaScript:

1. Use the Number.isFinite() method to determine whether the passed value is a number.
2. The Number.isFinite() method returns true if the passed value is not positive Infinity, negative Infinity, or NaN.

Number.isFinite(34)         // true
Number.isFinite(2e64)       // true

Number.isFinite('0')        // false
Number.isFinite(null)       // false
Number.isFinite({})         // false

Number.isFinite(Infinity)   // false
Number.isFinite(NaN)        // false
Number.isFinite(-Infinity)  // false

The Number.isFinite() method was introduced in ES6 to check if the given value is a finite number. It returns true if the passed value is a number, otherwise false.

JavaScript also provides a global isFinite() function that converts the parameter to a Number and returns true if the resulting value is not positive or negative Infinity, NaN, or undefined.

This means, with Number.isFinite(), only values of the type number and are finite return true, and non-numbers always return false:

isFinite('2')             // true
Number.isFinite('2')      // false

isFinite(null)            // true
Number.isFinite(null)     // false

Alternatively, you can use the Number.isNaN() method to check if the passed value is a Number and NaN (Not a Number). This method is a more robust replacement of the global isNaN() function in JavaScript.

Number.isNaN(34)         // false <-- 34 is a valid number
Number.isNaN(2e64)       // false

Number.isNaN('0')        // false <-- 0 is a valid number
Number.isNaN(null)       // false (false positive?)
Number.isNaN({})         // false

Number.isNaN(Infinity)   // false
Number.isNaN(NaN)        // true <-- NaN is "Not a Number"
Number.isNaN(-Infinity)  // false

Unlike the global isNaN() function, Number.isNaN() does not convert the argument to a Number; it returns true when the argument is a Number and is NaN.

Finally, the typeof operator can also be used to check if the variable type is a number in JavaScript:

typeof 3 === 'number'       // true
typeof '0' === 'number'     // false ("string" type)

typeof null === 'number'          // false
typeof {} === 'number'            // false
typeof undefined === 'number'     // false

typeof NaN === 'number'         // true (false positive?) 
typeof Infinity === 'number'    // true

The typeof operator is more robust than Number.isNaN() and it correctly identifies null and undefined are not numbers.

However, it returns true for Infinity and NaN because these are special number values in JavaScript.

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