Check if an object is a Promise

Detect a thenable: an object or function with a then method.

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

function isPromise(value) { return value !== null && (typeof value === 'object' || typeof value === 'function') && typeof value.then === 'function'; }

Usage

isPromise(Promise.resolve('sent')); // → true
isPromise({ then() {} }); // → true
isPromise(async () => {}); // → false
isPromise(null); // → 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

• This detects thenables, which is what await and Promise.resolve accept; a plain object with then counts, as the second line shows.
• To normalize instead of testing, write await value or Promise.resolve(value).

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

• Check whether all values are equal — Compare every item with the first using Object.is.
• Check whether an object is empty — Check for own enumerable string keys on a JSON object.
• Check whether an integer is even — Test parity for safe integers, including zero and negative values.