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How to Check if a Number is a Float or an Integer in JavaScript

When working with numbers in JavaScript, it's often necessary to distinguish between an integer (a whole number, like 10) and a floating-point number (a number with a decimal part, like 10.5). This is a common requirement for validation, mathematical calculations, or for handling data from an API.

This guide will teach you how to use the modern, built-in Number.isInteger() method as the primary tool for this task. You will learn how to create simple, reusable functions to reliably check if a value is an integer or a float.

The Core Method: Number.isInteger()​

The Number.isInteger() method is the standard and most reliable way to determine if a value is a whole number.

The logic:

  • It returns true if the value is an integer.
  • It returns false for everything else, including floats, strings, NaN, and null.

For example:

console.log(Number.isInteger(42));        // Output: true
console.log(Number.isInteger(-100));      // Output: true
console.log(Number.isInteger(42.0));      // Output: true (because it has no fractional part)

console.log(Number.isInteger(42.5));      // Output: false
console.log(Number.isInteger('42'));      // Output: false (it's a string)
console.log(Number.isInteger(Infinity));  // Output: false
note

The key takeaway is that Number.isInteger() is a very strict and predictable check.

Solution 1: Check if a Value is an Integer​

Using the Number.isInteger() method directly is the best way to check for an integer. For robustness, you can wrap it in a function.

function isInteger(value) {
  return Number.isInteger(value);
}

console.log(isInteger(123));   // Output: true
console.log(isInteger(12.0));  // Output: true
console.log(isInteger(12.3));  // Output: false
console.log(isInteger('123')); // Output: false
note

This is the simplest and most correct way to check if a value is an integer.

Solution 2: Check if a Value is a Float​

There is no built-in Number.isFloat() method in JavaScript. To check for a float, we must use a combination of checks.

The logic: A value is a float if it meets all of the following three conditions:

  1. Its typeof must be 'number'.
  2. It must not be NaN (Not-a-Number).
  3. It must not be an integer (Number.isInteger() must be false).

Solution:

function isFloat(value) {
  if (
    typeof value === 'number' &&
    !Number.isNaN(value) &&
    !Number.isInteger(value)
  ) {
    return true;
  }

  return false;
}

// ✅ Correct results for floats
console.log(isFloat(12.34));   // Output: true
console.log(isFloat(-0.5));    // Output: true

// ⛔️ Correct results for non-floats
console.log(isFloat(12.0));    // Output: false (is an integer)
console.log(isFloat(12));      // Output: false (is an integer)
console.log(isFloat('12.3'));  // Output: false (is a string)
console.log(isFloat(NaN));     // Output: false

How the Methods Work and Their Key Differences​

  • Number.isInteger(): This is a static method on the Number object that performs a strict check. It does not perform any type coercion, which is why Number.isInteger('42') is false. It's a reliable and precise tool.
  • isFloat() Function: Our custom function works by a process of elimination. The typeof value === 'number' is a broad check that allows numbers, NaN, and Infinity. We then use !Number.isNaN() and !Number.isInteger() to narrow it down, leaving only the true floating-point numbers.

A Note on Performance and Readability​

For this common task, the built-in Number.isInteger() and the custom isFloat() function are extremely performant. They are also the most readable and universally understood solutions in modern JavaScript. While you could use a regular expression to check if a string representation of a number contains a dot, that approach is less direct and doesn't work on number types without first converting them to strings. The methods presented here are the best practice.

Conclusion​

Distinguishing between integers and floats is a straightforward task in JavaScript with the right built-in tools.

The key takeaways are:

  1. To check if a value is an integer, use the built-in Number.isInteger(value) method. It is the most reliable and direct approach.
  2. To check if a value is a float, you must use a combination of checks: the value must be a number, must not be NaN, and must not be an integer.
  3. Be aware that Number.isInteger(10.0) is true, as the value has no fractional part.

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