The Promise.resolve()
method returns a Promise
object that is resolved with a given value. If the value is a promise, that promise is returned; if the value is a thenable (i.e. has a "then" method
), the returned promise will "follow" that thenable, adopting its eventual state; otherwise the returned promise will be fulfilled with the value. This function flattens nested layers of promise-like objects (e.g. a promise that resolves to a promise that resolves to something) into a single layer.
Syntax
Promise.resolve(value);
Parameters
value
- Argument to be resolved by this
Promise
. Can also be aPromise
or a thenable to resolve.
Return value
A Promise
that is resolved with the given value, or the promise passed as value, if the value was a promise object.
Description
The static Promise.resolve
function returns a Promise
that is resolved.
Examples
Using the static Promise.resolve method
Promise.resolve('Success').then(function(value) { console.log(value); // "Success" }, function(value) { // not called });
Resolving an array
var p = Promise.resolve([1,2,3]); p.then(function(v) { console.log(v[0]); // 1 });
Resolving another Promise
var original = Promise.resolve(33); var cast = Promise.resolve(original); cast.then(function(value) { console.log('value: ' + value); }); console.log('original === cast ? ' + (original === cast)); // logs, in order: // original === cast ? true // value: 33
The inverted order of the logs is due to the fact that the then
handlers are called asynchronously. See how then
works here.
Resolving thenables and throwing Errors
// Resolving a thenable object var p1 = Promise.resolve({ then: function(onFulfill, onReject) { onFulfill('fulfilled!'); } }); console.log(p1 instanceof Promise) // true, object casted to a Promise p1.then(function(v) { console.log(v); // "fulfilled!" }, function(e) { // not called }); // Thenable throws before callback // Promise rejects var thenable = { then: function(resolve) { throw new TypeError('Throwing'); resolve('Resolving'); }}; var p2 = Promise.resolve(thenable); p2.then(function(v) { // not called }, function(e) { console.error(e); // TypeError: Throwing }); // Thenable throws after callback // Promise resolves var thenable = { then: function(resolve) { resolve('Resolving'); throw new TypeError('Throwing'); }}; var p3 = Promise.resolve(thenable); p3.then(function(v) { console.log(v); // "Resolving" }, function(e) { // not called });
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Promise.resolve' in that specification. |
Browser compatibility
To contribute to this compatibility data, please write a pull request against this repository: https://github.com/mdn/browser-compat-data.
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
resolve() | Chrome Full support 32 | Edge Full support 12 | Firefox Full support 29 | IE No support No | Opera Full support 19 | Safari Full support 8 | WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android Full support 29 | Opera Android Full support 19 | Safari iOS Full support 8 | Samsung Internet Android Full support 2.0 | nodejs Full support 0.12 |
Legend
- Full support
- Full support
- No support
- No support