Number.isInteger()

Number.isInteger(value)

Returns: Boolean · Updated March 13, 2026 · Number

javascript number integer type-check

The Number.isInteger() method determines whether the passed value is an integer. This is useful for validating user input, checking API responses, or ensuring calculations produce whole numbers. In JavaScript, integers are numbers without a fractional component, and this method provides a reliable way to test for them.

Syntax

Number.isInteger(value)

Parameters

Parameter	Type	Description
`value`	any	The value to check

Return Value

Returns true if the value is an integer (a whole number without any fractional component). Returns false for non-integers, non-number types, and special values like Infinity, -Infinity, and NaN.

Examples

Basic Usage

Number.isInteger(42);       // true
Number.isInteger(0);        // true
Number.isInteger(-10);      // true
Number.isInteger(1000);     // true

Floating Point Numbers

Floating point numbers return false because they have a fractional component:

Number.isInteger(3.14159);  // false
Number.isInteger(1.5);      // false
Number.isInteger(0.1);      // false
Number.isInteger(-2.5);     // false

Edge Cases: Integer-Like Floats

Interestingly, JavaScript treats some floating point numbers as integers:

// These are technically floats but are considered integers by isInteger
Number.isInteger(1.0);       // true (1.0 equals 1)
Number.isInteger(42.0);      // true
Number.isInteger(-5.0);     // true

This is because JavaScript doesn’t distinguish between 1 and 1.0 at the language level — they’re both the integer value 1.

Special Values

Special numeric values return false:

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

Non-Number Types

Non-number types always return false:

Number.isInteger('42');       // false (string)
Number.isInteger('hello');    // false
Number.isInteger(undefined);  // false
Number.isInteger(null);       // false
Number.isInteger(true);       // false (boolean)
Number.isInteger(false);      // false
Number.isInteger({});         // false (object)
Number.isInteger([]);         // false (array)

Common Patterns

Form Validation

Validating that user input is a whole number:

function validateAge(input) {
  if (!Number.isInteger(input)) {
    return 'Age must be a whole number';
  }
  if (input < 0 || input > 150) {
    return 'Age must be between 0 and 150';
  }
  return 'Valid';
}

validateAge(25);    // 'Valid'
validateAge(25.5); // 'Age must be a whole number'
validateAge(-5);   // 'Age must be between 0 and 150'

Array Index Validation

Ensuring an index is a valid integer:

function getItem(arr, index) {
  if (!Number.isInteger(index)) {
    throw new Error('Index must be an integer');
  }
  if (index < 0 || index >= arr.length) {
    throw new Error('Index out of bounds');
  }
  return arr[index];
}

const items = ['a', 'b', 'c'];
getItem(items, 1);    // 'b'
getItem(items, 1.5);  // Error: Index must be an integer

Loop Counter Validation

function createArray(length) {
  if (!Number.isInteger(length) || length <= 0) {
    throw new Error('Length must be a positive integer');
  }
  return new Array(length).fill(0);
}

createArray(5);    // [0, 0, 0, 0, 0]
createArray(3.5); // Error: Length must be a positive integer

Type Coercion Comparison

Unlike some other methods, Number.isInteger() does not coerce its argument:

// Comparison with the global approach ( Number() conversion)
Number.isInteger(Number('42'));  // true (after coercion to 42)
Number.isInteger('42');          // false (string, not coerced)

Number.isInteger(Number('3.14')); // false (after coercion to 3.14)
Number.isInteger('3.14');        // false (string)

Filtering Arrays

const mixed = [1, 2.5, 3, 4.5, 5, 6, 7.5];
const integers = mixed.filter(Number.isInteger);
// [1, 3, 5, 6]

const floats = mixed.filter(x => !Number.isInteger(x));
// [2.5, 4.5, 7.5]

Combined Validation

function processQuantity(value) {
  if (typeof value !== 'number' || !Number.isFinite(value)) {
    return { error: 'Must be a finite number' };
  }
  if (!Number.isInteger(value)) {
    return { error: 'Must be an integer' };
  }
  if (value < 0) {
    return { error: 'Must be non-negative' };
  }
  return { success: true, value };
}

processQuantity(10);     // { success: true, value: 10 }
processQuantity(10.5);   // { error: 'Must be an integer' }
processQuantity(Infinity); // { error: 'Must be a finite number' }