Check if a value is an async function

Read the internal tag of async function declarations and arrows.

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

function isAsyncFunction(value) { return Object.prototype.toString.call(value) === '[object AsyncFunction]'; }

Usage

isAsyncFunction(async () => {}); // → true
isAsyncFunction(() => Promise.resolve()); // → false
isAsyncFunction(function* () {}); // → 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

• A regular function that returns a promise is not detected, as the second line shows, and code transpiled to older JavaScript loses the tag.
• Do not branch on it: await the result, which works for both.

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.