How to determine if a Promise is supported by the browser

Question

Does anyone know, using Modernizr or otherwise, if there is a way to detect if the Promise feature is enabled in a browser?

I have a polyfill for the functionality, but only want to apply it if the browser does not have a native implementation.

Right here: http://caniuse.com/#search=promise – Diodeus - James MacFarlane Mar 19 '14 at 20:09 — Diodeus - James MacFarlane, Mar 19 '14 at 20:09
`var supported = ('Promise' in window);` – Whymarrh Mar 19 '14 at 20:10 — Whymarrh, Mar 19 '14 at 20:10

Benjamin Gruenbaum · Accepted Answer · 2016-12-11T12:05:52.257

44

Update Dec 11 - 2016: All evergreen versions of browsers now support promises. They are safe to use.

Update Nov 14 - 2016: Chrome, Firefox, Safari and IE all now have experimental support for promises in their dev channels. The specification has settled. I would still not rely on the implementation just yet and would use a library but this might change in a few months.

No browsers support promises natively in a reliable way. The specification might change - at least for a few more months. My suggestion is use a fast promise library like Bluebird.

If you want to check if native promises are enabled - you can do :

if(typeof Promise !== "undefined" && Promise.toString().indexOf("[native code]") !== -1){
    //here
}

As others suggested, just checking if there is a Promise object can be done by if(Promise) but I strongly suggest against it since different libraries have different APIs for creation of promises etc.

edited Dec 11 '16 at 12:05

answered Mar 19 '14 at 20:16

Benjamin Gruenbaum

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Shouldn't that be `typeof Promise !== "undefined"`, since `typeof` returns a string? – Paul Roub Mar 19 '14 at 20:38
@PaulRoub yeah, it started as `Promise !== undefined` but I realized the identifier might not be declared. – Benjamin Gruenbaum Mar 19 '14 at 21:49
This is particularly useful if you want to know if the `Promise` you're working with is native or a polyfill. Unfortunately, many polyfills (including the one used by Traceur) do not set a flag to indicate that they aren't native. – Bailey Parker Jun 08 '15 at 12:28
@PhpMyCoder why would you care though? – Benjamin Gruenbaum Jun 08 '15 at 12:31
Native Promises (well native `Promise.all()`) + Traceur generators are currently broken: https://github.com/google/traceur-compiler/issues/1955 – Bailey Parker Jun 08 '15 at 13:50
What would be the reverse of this? `===` undefined and `=== -1`? – Floris Nov 12 '18 at 08:29

King Friday · Answer 2 · 2021-07-29T22:53:30.877

16

Not so fast.

This throws when "Promise" is undefined:

if (Promise)
  // do code

This never throws:

if (window.Promise)
  // do code

if (typeof Promise !== 'undefined')
  // do code

and yes the window object can be relied upon in a browser environment.

edited Jul 29 '21 at 22:53

answered Mar 26 '16 at 04:07

King Friday

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`Object.defineProperty(window, 'Promise', { get() { throw Error(); }});` – Benjamin Gruenbaum Mar 16 '18 at 15:12

score 6 · Answer 3 · answered Mar 19 '14 at 20:14

6

To create a 'supported' flag without carrying around an object reference:

var canPromise = !! window.Promise;

answered Mar 19 '14 at 20:14

Paul Roub

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score 3 · Answer 4 · answered Mar 19 '14 at 20:20

3

You could try to create one in a try/catch block:

var promiseSupport = false;
try {
    var promise = new Promise(function (x, y) {});
    promiseSupport = true;
} catch (e) {}

Check promiseSupport to see whether or not it fails.

JSFiddle

answered Mar 19 '14 at 20:20

War10ck

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How to determine if a Promise is supported by the browser

4 Answers4

Not so fast.

Linked