The Number isInteger() method

Number.isInteger() returns true if the value you pass 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

Why do we need it?

Before this method arrived with ES2015, checking for an integer meant writing something like typeof value === 'number' && value % 1 === 0. That works, but it’s easy to get wrong.

Number.isInteger() does the check in one call. A common use is validating a quantity before using it:

const quantity = 3
if (Number.isInteger(quantity)) {
  //safe to use as an item count
}

It does not convert values

The method never converts what you pass to a number. The string '5' is not an integer, it’s a string:

Number.isInteger('5') //false
Number.isInteger(5) //true

This is the big difference from a check like value % 1 === 0, where JavaScript would convert '5' to 5 first and give you true.

What about edge cases?

A number with a fractional part of zero counts as an integer:

Number.isInteger(5.0) //true
Number.isInteger(5e3) //true, it's 5000

NaN and Infinity are numbers, but not integers:

Number.isInteger(NaN) //false
Number.isInteger(Infinity) //false

Very large numbers can surprise you. JavaScript can’t store every huge value precisely, so some decimals round to the nearest integer before the check runs:

Number.isInteger(5.0000000000000001) //true

The literal becomes 5 before isInteger() ever sees it. If you work with numbers that big, look at Number.isSafeInteger(), which also checks the value is small enough to be stored exactly.

BigInt values fail the check too, even when they hold whole numbers. A BigInt is a different type, not a number:

Number.isInteger(10n) //false

A common pitfall with form input

Values coming from an HTML input are always strings. This check fails even when the user typed a whole number:

const input = '42'
Number.isInteger(input) //false

Convert first, then check:

Number.isInteger(Number(input)) //true

Number('42') gives you 42, and the check passes. Skip the conversion and every value from a form will fail validation.