The JavaScript Number.isFinite() method

Q: What does Number.isFinite() check?

Whether a value is a finite number — it returns false for Infinity, -Infinity, NaN and non-numbers.

Q: What is the difference between Number.isFinite() and the global isFinite()?

The global isFinite() coerces its argument first, so isFinite("42") is true. Number.isFinite() doesn't coerce and only accepts real numbers.

Q: How do I check for Infinity?

Use !Number.isFinite(x) && !Number.isNaN(x), or compare directly with Infinity/-Infinity.

Q: When would a value be Infinity?

From operations like division by zero (1 / 0) or numbers that overflow the range JavaScript can represent.

Method JavaScript All modern browsers Updated June 23, 2026

Quick answer

The Number.isFinite() method returns true only if a value is a finite number — not Infinity, -Infinity or NaN, and not a non-number. Number.isFinite(42) is true; Number.isFinite("42") is false (no coercion). It's safer than the global isFinite().

Overview

Number.isFinite() confirms a value is a real, usable, finite number. It rules out the three problem values that arithmetic can produce — Infinity, -Infinity and NaN — and, crucially, it does not coerce, so anything that isn't already a number (like a string) returns false.

That no-coercion behavior is the difference from the older global isFinite(), which converts its argument first: isFinite("42") is true because the string converts to a number, which is often not what you want. Number.isFinite("42") is false — it only accepts actual numbers. Use the Number. version for reliable validation.

It's the natural guard after arithmetic that might overflow or divide by zero (1 / 0 is Infinity) or fail (parseInt("x") is NaN). It sits with the other strict numeric checks — Number.isNaN() and Number.isInteger() — that all avoid coercion. Use isFinite when "is this a normal number I can safely use" is the question.

Syntax

Number.isFinite(value)

Number.isFinite(42)        // true
Number.isFinite(Infinity)  // false
Number.isFinite(NaN)       // false
Number.isFinite("42")      // false (no coercion)
isFinite("42")             // true  (global, coerces - avoid)

Parameters

The Number.isFinite() method accepts the following parameters.

Parameter	Description
`value`	The value to test. Returns `true` only if it is a finite number.

Example

Live example

<pre id="out" style="font:15px ui-monospace,monospace"></pre>
<script>
  const values = [42, 1 / 0, NaN, '42', -3.14];

  const lines = values.map(v =>
    JSON.stringify(v) + ' -> ' + Number.isFinite(v)
  );

  document.getElementById('out').textContent = lines.join('\n');
</script>

Best practices

Use Number.isFinite() (not the global isFinite()) to avoid coercion.
Guard results of division and parsing, which can produce Infinity or NaN.
Pair it with Number.isNaN() and Number.isInteger() for thorough numeric checks.
Convert strings to numbers first if you need to accept numeric strings.

Frequently asked questions

What does Number.isFinite() check?

Whether a value is a finite number — it returns false for Infinity, -Infinity, NaN and non-numbers.

What is the difference between Number.isFinite() and the global isFinite()?

The global isFinite() coerces its argument first, so isFinite("42") is true. Number.isFinite() doesn't coerce and only accepts real numbers.

How do I check for Infinity?

Use !Number.isFinite(x) && !Number.isNaN(x), or compare directly with Infinity/-Infinity.

When would a value be Infinity?

From operations like division by zero (1 / 0) or numbers that overflow the range JavaScript can represent.

Math.trunc() Number.isInteger()

References