Promise.all

The Promise.all(iterable) method returns a single Promise that resolves when all of the promises in the iterable argument have resolved or when the iterable argument contains no promises. It rejects with the reason of the first promise that rejects.

Syntax

Promise.all(iterable);

Parameters

iterable

An iterable object such as an Array or String.

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.

Fulfillment:
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.
In all cases, the returned promise is fulfilled with an array containing all the values of the iterable passed as argument (also non-promise values).

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.log(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_resolvement'), 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_resolvement"
  console.log(values[1]) // "Error: p2_immediate_rejection"
})

Specifications

Browser compatibilityUpdate compatibility data on GitHub

See also

• Promise
• Promise.race()