JavaScript Number.isInteger()

Number.isInteger(value) returns true when value is a JavaScript number with no fractional part. It does not convert strings to numbers.

console.log(Number.isInteger(5));     // true
console.log(Number.isInteger(5.5));   // false
console.log(Number.isInteger("5"));   // false

Show Table of Contents ↓

Number.isInteger() syntax Top ↑

Number.isInteger(value)

The method returns a Boolean and performs no type coercion.

Positive, negative and decimal examples Top ↑

console.log(Number.isInteger(5));      // true
console.log(Number.isInteger(-5));     // true
console.log(Number.isInteger(0));      // true
console.log(Number.isInteger(5.5));    // false
console.log(Number.isInteger(-5.5));   // false
console.log(Number.isInteger(NaN));    // false
console.log(Number.isInteger(Infinity)); // false

Why Number.isInteger(5.0) is true Top ↑

JavaScript has one ordinary Number type for both integers and floating-point values. 5 and 5.0 represent the same numeric value.

console.log(Number.isInteger(5.0)); // true

Numeric strings are not integers until converted Top ↑

const raw = "42";
console.log(Number.isInteger(raw));         // false
const value = Number(raw);
console.log(Number.isInteger(value));       // true

Number.isInteger() vs Number.isSafeInteger() Top ↑

Number.isInteger() checks whether a Number has an integer value. Number.isSafeInteger() additionally checks whether that integer can be represented exactly within JavaScript's safe-integer range.

const n = Number.MAX_SAFE_INTEGER + 1;
console.log(Number.isInteger(n));     // true
console.log(Number.isSafeInteger(n)); // false

Floating-point precision can affect the result Top ↑

Because JavaScript Numbers use binary floating-point representation, a value that looks fractional in source code can sometimes round to an integer at very large magnitudes. Treat Number.isInteger() as a check of the stored Number value, not a guarantee about the original text entered by a user.

Validate an integer entered in a form Top ↑

function readInteger(raw) {
  if (raw.trim() === "") return null;
  const value = Number(raw);
  return Number.isInteger(value) ? value : null;
}

Related tutorials Top ↑

See NaN, numeric validation, and factors exercise.

← JavaScript Math Reference

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

About the author   Watch on YouTube

Connect: