Check if a value is a function

True for anything callable: functions, arrows, async and generator functions, classes.

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

function isFunction(value) { return typeof value === 'function'; }

Usage

isFunction(function () {}); // → true
isFunction(async () => {}); // → true
isFunction(function* () {}); // → true
isFunction(class Mailer {}); // → true
isFunction(Promise.resolve(1)); // → false

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• The original also returned true for a Promise, which is not callable. typeof is the reliable test, as the last line shows.
• Classes are functions too: typeof cannot tell whether calling without new will throw.

The function above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Logical xor operator — True when exactly one of two values is truthy.
• Identity function — Return the argument unchanged.
• Box handler — Chain transformations on a value, then unwrap the result.