JavaScript Promises Waiting for multiple concurrent promises
Example
The Promise.all() static method accepts an iterable (e.g. an Array) of promises and returns a new promise, which resolves when all promises in the iterable have resolved, or rejects if at least one of the promises in the iterable have rejected.
// wait "millis" ms, then resolve with "value"
function resolve(value, milliseconds) {
return new Promise(resolve => setTimeout(() => resolve(value), milliseconds));
}
// wait "millis" ms, then reject with "reason"
function reject(reason, milliseconds) {
return new Promise((_, reject) => setTimeout(() => reject(reason), milliseconds));
}
Promise.all([
resolve(1, 5000),
resolve(2, 6000),
resolve(3, 7000)
]).then(values => console.log(values)); // outputs "[1, 2, 3]" after 7 seconds.
Promise.all([
resolve(1, 5000),
reject('Error!', 6000),
resolve(2, 7000)
]).then(values => console.log(values)) // does not output anything
.catch(reason => console.log(reason)); // outputs "Error!" after 6 seconds.
Non-promise values in the iterable are "promisified".
Promise.all([
resolve(1, 5000),
resolve(2, 6000),
{ hello: 3 }
])
.then(values => console.log(values)); // outputs "[1, 2, { hello: 3 }]" after 6 seconds
Destructuring assignment can help to retrieve results from multiple promises.
Promise.all([
resolve(1, 5000),
resolve(2, 6000),
resolve(3, 7000)
])
.then(([result1, result2, result3]) => {
console.log(result1);
console.log(result2);
console.log(result3);
});
