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Promise.resolve()

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 a Promise 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
});

Warning: Do not call Promise.resolve() on a thenable that resolves to itself. That leads to infinite recursion, because it attempts to flatten an infinitely-nested promise.

let thenable = {
  then: (resolve, reject) => {
    resolve(thenable)
  }
}

Promise.resolve(thenable)  // Will lead to infinite recursion.

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
resolve
32
12
29
No
19
8
4.4.3
32
29
19
8
2.0
1.0
0.12.0

See also

© 2005–2021 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/resolve