The Promise.all()
method takes an iterable of promises as an input, and returns a single Promise
that resolves to an array of the results of the input promises. This returned promise will resolve when all of the input's promises have resolved, or if the input iterable contains no promises. It rejects immediately upon any of the input promises rejecting or non-promises throwing an error, and will reject with this first rejection message / error.
The source for this interactive demo is stored in a GitHub repository. If you'd like to contribute to the interactive demo project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
Syntax
Promise.all(iterable);
Parameters
Return value
- An already resolved
Promise
if the iterable passed is empty. - An asynchronously resolved
Promise
if the iterable passed contains no promises. Note, Google Chrome 58 returns an already resolved promise in this case. - A pending
Promise
in all other cases. This returned promise is then resolved/rejected asynchronously (as soon as the stack is empty) when all the promises in the given iterable have resolved, or if any of the promises reject. See the example about "Asynchronicity or synchronicity of Promise.all" below. Returned values will be in order of the Promises passed, regardless of completion order.
Description
This method can be useful for aggregating the results of multiple promises. It is typically used when there are multiple asynchronous tasks that are dependent on one another to complete successfully, as it does not wait and will reject immediately upon any of the input promises rejecting.
In comparison, the promise returned by Promise.allSettled()
will wait for all input promises to complete, regardless of whether or not one rejects. Consequently, it will always return the final result of every promise and function from the input iterable.
Fulfillment
The returned promise is fulfilled with an array containing all the values of the iterable passed as argument (also non-promise values).
- If an empty iterable is passed, then this method returns (synchronously) an already resolved promise.
- If all of the passed-in promises fulfill, or are not promises, the promise returned by
Promise.all
is fulfilled asynchronously.
Rejection
If any of the passed-in promises reject, Promise.all
asynchronously rejects with the value of the promise that rejected, whether or not the other promises have resolved.
Examples
Using Promise.all
Promise.all
waits for all fulfillments (or the first rejection).
var p1 = Promise.resolve(3); var p2 = 1337; var p3 = new Promise((resolve, reject) => { setTimeout(() => { resolve("foo"); }, 100); }); Promise.all([p1, p2, p3]).then(values => { console.log(values); // [3, 1337, "foo"] });
If the iterable contains non-promise values, they will be ignored, but still counted in the returned promise array value (if the promise is fulfilled):
// this will be counted as if the iterable passed is empty, so it gets fulfilled var p = Promise.all([1,2,3]); // this will be counted as if the iterable passed contains only the resolved promise with value "444", so it gets fulfilled var p2 = Promise.all([1,2,3, Promise.resolve(444)]); // this will be counted as if the iterable passed contains only the rejected promise with value "555", so it gets rejected var p3 = Promise.all([1,2,3, Promise.reject(555)]); // using setTimeout we can execute code after the stack is empty setTimeout(function() { console.log(p); console.log(p2); console.log(p3); }); // logs // Promise { <state>: "fulfilled", <value>: Array[3] } // Promise { <state>: "fulfilled", <value>: Array[4] } // Promise { <state>: "rejected", <reason>: 555 }
Asynchronicity or synchronicity of Promise.all
This following example demonstrates the asynchronicity (or synchronicity, if the iterable passed is empty) of Promise.all
:
// we are passing as argument an array of promises that are already resolved, // to trigger Promise.all as soon as possible var resolvedPromisesArray = [Promise.resolve(33), Promise.resolve(44)]; var p = Promise.all(resolvedPromisesArray); // immediately logging the value of p console.log(p); // using setTimeout we can execute code after the stack is empty setTimeout(function() { console.log('the stack is now empty'); console.log(p); }); // logs, in order: // Promise { <state>: "pending" } // the stack is now empty // Promise { <state>: "fulfilled", <value>: Array[2] }
The same thing happens if Promise.all
rejects:
var mixedPromisesArray = [Promise.resolve(33), Promise.reject(44)]; var p = Promise.all(mixedPromisesArray); console.log(p); setTimeout(function() { console.log('the stack is now empty'); console.log(p); }); // logs // Promise { <state>: "pending" } // the stack is now empty // Promise { <state>: "rejected", <reason>: 44 }
But, Promise.all
resolves synchronously if and only if the iterable passed is empty:
var p = Promise.all([]); // will be immediately resolved var p2 = Promise.all([1337, "hi"]); // non-promise values will be ignored, but the evaluation will be done asynchronously console.log(p); console.log(p2) setTimeout(function() { console.log('the stack is now empty'); console.log(p2); }); // logs // Promise { <state>: "fulfilled", <value>: Array[0] } // Promise { <state>: "pending" } // the stack is now empty // Promise { <state>: "fulfilled", <value>: Array[2] }
Promise.all
fail-fast behaviour
Promise.all
is rejected if any of the elements are rejected. For example, if you pass in four promises that resolve after a timeout and one promise that rejects immediately, then Promise.all
will reject immediately.
var p1 = new Promise((resolve, reject) => { setTimeout(() => resolve('one'), 1000); }); var p2 = new Promise((resolve, reject) => { setTimeout(() => resolve('two'), 2000); }); var p3 = new Promise((resolve, reject) => { setTimeout(() => resolve('three'), 3000); }); var p4 = new Promise((resolve, reject) => { setTimeout(() => resolve('four'), 4000); }); var p5 = new Promise((resolve, reject) => { reject(new Error('reject')); }); // Using .catch: Promise.all([p1, p2, p3, p4, p5]) .then(values => { console.log(values); }) .catch(error => { console.error(error.message) }); //From console: //"reject"
It is possible to change this behaviour by handling possible rejections:
var p1 = new Promise((resolve, reject) => { setTimeout(() => resolve('p1_delayed_resolution'), 1000); }); var p2 = new Promise((resolve, reject) => { reject(new Error('p2_immediate_rejection')); }); Promise.all([ p1.catch(error => { return error }), p2.catch(error => { return error }), ]).then(values => { console.log(values[0]) // "p1_delayed_resolution" console.error(values[1]) // "Error: p2_immediate_rejection" })
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Promise.all' 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 | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
all() | 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