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.

Solution 1

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

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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.

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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.

Solution 2

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.

Solution 3

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

var canPromise = !! window.Promise;

Solution 4

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

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Comments

[Andrew Prock]

Shouldn't that be typeof Promise !== "undefined", since typeof returns a string?

[Benjamin Gruenbaum]

@PaulRoub yeah, it started as Promise !== undefined but I realized the identifier might not be declared.

[Bailey Parker]

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.

[Benjamin Gruenbaum]

@PhpMyCoder why would you care though?

[Bailey Parker]

Native Promises (well native Promise.all()) + Traceur generators are currently broken: github.com/google/traceur-compiler/issues/1955

[Benjamin Gruenbaum]

Object.defineProperty(window, 'Promise', { get() { throw Error(); }});

[Floris]

What would be the reverse of this? === undefined and === -1?