How do I tell if an object is a Promise?

Checking if something is promise unnecessarily complicates the code, just use Promise.resolve

Promise.resolve(valueOrPromiseItDoesntMatter).then(function(value) { }) 

Disclaimer: this is not a good answer to the updated question, is per-library, and won't work across realms. Check for .then instead.

This answer, based on the spec is a way to test for a promise that works only sometimes, FYI.

Promise.resolve(obj) == obj && BLUEBIRD.resolve(obj) == obj 

When this works it's because the algorithm explicitly demands that Promise.resolve must return the exact object passed in if and only if it is a promise created by this constructor.

Disclaimer: this is not a good answer to the updated question; works for native only, and not across realms. Follow the accepted answer instead.

obj instanceof Promise 

should do it. Note that this may only work reliably with native es6 promises.

If you're using a shim, a promise library or anything else pretending to be promise-like, then it may be more appropriate to test for a "thenable" (anything with a .then method), as shown in other answers here.

if (typeof thing?.then === 'function') { // probably a promise } else { // definitely not a promise } 

To see if the given object is a ES6 Promise, we can make use of this predicate:

function isPromise(p) { return p && Object.prototype.toString.call(p) === "[object Promise]"; } 

Calling toString directly from the Object.prototype returns a native string representation of the given object type which is "[object Promise]" in our case. This ensures that the given object

• Bypasses false positives such as..:
  • Self-defined object type with the same constructor name ("Promise").
  • Self-written toString method of the given object.
• Works across multiple environment contexts (e.g. iframes) in contrast to instanceof or isPrototypeOf.

However, any particular host object, that has its tag modified via Symbol.toStringTag, can return "[object Promise]". This may be the intended result or not depending on the project (e.g. if there is a custom Promise implementation).

To see if the object is from a native ES6 Promise, we can use:

function isNativePromise(p) { return p && typeof p.constructor === "function" && Function.prototype.toString.call(p.constructor).replace(/\(.*\)/, "()") === Function.prototype.toString.call(/*native object*/Function) .replace("Function", "Promise") // replacing Identifier .replace(/\(.*\)/, "()"); // removing possible FormalParameterList } 

According to this and this section of the spec, the string representation of function should be:

"function Identifier ( FormalParameterList_opt ) { FunctionBody }"

which is handled accordingly above. The FunctionBody is [native code] in all major browsers.

MDN: Function.prototype.toString

This works across multiple environment contexts as well.

This is how graphql-js package detects promises:

function isPromise(value) { return Boolean(value && typeof value.then === 'function'); } 

value is the returned value of your function. I'm using this code in my project and have no problem so far.

Update 2025:

Here is the up-to-date code:

export function isPromise(value: any): value is Promise<unknown> { return typeof value?.then === 'function'; } 

Not an answer to the full question but I think it's worth to mention that in Node.js 10 a new util function called isPromise was added which checks if an object is a native Promise or not:

const utilTypes = require('util').types const b_Promise = require('bluebird') utilTypes.isPromise(Promise.resolve(5)) // true utilTypes.isPromise(b_Promise.resolve(5)) // false 

If you are in an async method you can do this and avoid any ambiguity.

async myMethod(promiseOrNot){ const theValue = await promiseOrNot() } 

If the function returns promise, it will await and return with the resolved value. If the function returns a value, it will be treated as resolved.

If the function does not return a promise today, but tomorrow returns one or is declared async, you will be future-proof.

Here is the code from https://github.com/ssnau/xkit/blob/master/util/is-promise.js

!!obj && (typeof obj === 'object' || typeof obj === 'function') && typeof obj.then === 'function'; 

If an object has a then method, it should be treat as a Promise.

In case you are using Typescript, I'd like to add that you can use the "type predicate" feature. Just should wrap the logical verification in a function that returns x is Promise<any> and you won't need to do typecasts. Below on my example, c is either a promise or one of my types which I want to convert into a promise by calling the c.fetch() method.

export function toPromise(c: Container<any> | Promise<any>): Promise<any> { if (c == null) return Promise.resolve(); return isContainer(c) ? c.fetch() : c; } export function isContainer(val: Container<any> | Promise<any>): val is Container<any> { return val && (<Container<any>>val).fetch !== undefined; } export function isPromise(val: Container<any> | Promise<any>): val is Promise<any> { return val && (<Promise<any>>val).then !== undefined; } 

More info: https://www.typescriptlang.org/docs/handbook/advanced-types.html

Anything that pushes a possibly synch value into Promise.resolve(value) for the comfort of avoiding comparison turns your code into an otherwise avoidable async. Sometimes you don't want it at that stage. You want to know the result evaluated right before some earlier resolution in the microtask queue bites you right..?

One can possibly do like;

var isPromise = x => Object(x).constructor === Promise; 

I checked it against some edge cases that i can think of and it seems to work.

isPromise(undefined); // <- false isPromise(null); // <- false isPromise(0); // <- false isPromise(""); // <- false isPromise({}); // <- false isPromise(setTimeout); // <- false isPromise(Promise); // <- false isPromise(new Promise((v,x) => setTimeout(v,1000,"whatever"))); // <- true isPromise(fetch('http://example.com/movies.json')); // <- true 

I haven't checked it up against any non-native librarires but what's the point now?

I use this function as a universal solution:

function isPromise(value) { return value && value.then && typeof value.then === 'function'; } 

after searching for a reliable way to detect Async functions or even Promises, i ended up using the following test :

() => fn.constructor.name === 'Promise' || fn.constructor.name === 'AsyncFunction' 

For those trying to do this in Typescript - which errors with the other provided solutions:

if (p instanceof Object && 'then' in p && typeof p.then === 'function') { ... } 

it('should return a promise', function() { var result = testedFunctionThatReturnsPromise(); expect(result).toBeDefined(); // 3 slightly different ways of verifying a promise expect(typeof result.then).toBe('function'); expect(result instanceof Promise).toBe(true); expect(result).toBe(Promise.resolve(result)); }); 

const isPromise = (value) => { return !!( value && value.then && typeof value.then === 'function' && value?.constructor?.name === 'Promise' ) } 

As for me - this check is better, try it out

Into 2025, and without obsolete hooks...

• By checking the type of instance:

const isPromise = (value:any) => value instanceof Promise; 

• By checking for then function presence:

const isPromise = (value:any) => typeof value?.then === 'function'; 

Both work equally well today.