JavaScript Promises Waiting for the first of multiple concurrent promises
Example
The Promise.race() static method accepts an iterable of Promises and returns a new Promise which resolves or rejects as soon as the first of the promises in the iterable has resolved or rejected.
// wait "milliseconds" milliseconds, then resolve with "value"
function resolve(value, milliseconds) {
return new Promise(resolve => setTimeout(() => resolve(value), milliseconds));
}
// wait "milliseconds" milliseconds, then reject with "reason"
function reject(reason, milliseconds) {
return new Promise((_, reject) => setTimeout(() => reject(reason), milliseconds));
}
Promise.race([
resolve(1, 5000),
resolve(2, 3000),
resolve(3, 1000)
])
.then(value => console.log(value)); // outputs "3" after 1 second.
Promise.race([
reject(new Error('bad things!'), 1000),
resolve(2, 2000)
])
.then(value => console.log(value)) // does not output anything
.catch(error => console.log(error.message)); // outputs "bad things!" after 1 second
