JavaScript provides several methods to validate numbers and check their properties.

Number.isNaN()

NaN is a special case. A number is NaN only if it’s NaN or if it’s a division of 0 by 0 expression, which returns NaN. In all the other cases, we can pass it what we want but it will return false:

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

Number.isNaN(1) //false
Number.isNaN('Flavio') //false
Number.isNaN(true) //false
Number.isNaN({}) //false
Number.isNaN([1, 2, 3]) //false

Number.isFinite()

Returns true if the passed value is a finite number. Anything else, booleans, strings, objects, arrays, returns false:

Number.isFinite(1) //true
Number.isFinite(-237) //true
Number.isFinite(0) //true
Number.isFinite(0.2) //true

Number.isFinite('Flavio') //false
Number.isFinite(true) //false
Number.isFinite({}) //false
Number.isFinite([1, 2, 3]) //false

Number.isInteger()

Returns true if the passed value is an integer. Anything else, booleans, strings, objects, arrays, returns false:

Number.isInteger(1) //true
Number.isInteger(-237) //true
Number.isInteger(0) //true

Number.isInteger(0.2) //false
Number.isInteger('Flavio') //false
Number.isInteger(true) //false
Number.isInteger({}) //false
Number.isInteger([1, 2, 3]) //false

Number.isSafeInteger()

A number might satisfy Number.isInteger() but not Number.isSafeInteger() if it goes out of the boundaries of safe integers.

So, anything over 2^53 and below -2^53 is not safe:

Number.isSafeInteger(Math.pow(2, 53)) // false
Number.isSafeInteger(Math.pow(2, 53) - 1) // true
Number.isSafeInteger(Math.pow(2, 53) + 1) // false
Number.isSafeInteger(-Math.pow(2, 53)) // false
Number.isSafeInteger(-Math.pow(2, 53) - 1) // false
Number.isSafeInteger(-Math.pow(2, 53) + 1) // true

Checking if a Value is a Number

We have various ways to check if a value is a number.

The first is isNaN(), a global variable, assigned to the window object in the browser:

const value = 2

isNaN(value) //false

isNaN('test') //true

isNaN({}) //true

isNaN(1.2) //false

If isNaN() returns false, the value is a number.

Another way is to use the typeof operator. It returns the 'number' string if you use it on a number value:

typeof 1 //'number'

const value = 2

typeof value //'number'

So you can do a conditional check like this:

const value = 2
if (typeof value === 'number') {
  //it's a number
}

Lessons in this unit:

0:	Introduction
1:	Number Basics
2:	▶︎ Number Validation
3:	Parsing Numbers
4:	Formatting Numbers
5:	Number Recipes
6:	The Binary Number System
7:	Underscores in numbers
8:	The Decimal Number System
9:	Converting Numbers from Decimal to Binary