I want to declare an array of empty promises
then resolve them manually, but it is not working ?
const tab = [1]
let promises = tab.map(e => new Promise(() => { }))
setTimeout(() => {
promises[0] = Promise.resolve()
}, 3000);
Promise.all(promises)
.then(e => console.log("finished"))
I DO NOT WANT to do it like this :
const tab = [1]
let promises = tab.map(e => new Promise((resolve) => {
setTimeout(() => {
resolve()
}, 3000);
}))
Promise.all(promises)
.then(e => console.log("finished"))
Thats the closest you can get:
const tab = [1]
let resolves = []
let promises = tab.map(e => new Promise((resolve) => {
resolves.push(resolve)
}))
setTimeout(() => {
resolves[0]()
}, 3000);
Promise.all(promises).then(e => console.log("finished"))
Here's a technique using a reusable deferred function that allows you to externally control a promise. We extend both resolve and reject functionality, allowing better control for the caller. This is a somewhat unconventional pattern but there are advantages to exploiting promises in this way. See this Q&A for an detailed example -
function deferred () {
let resolve, reject, promise = new Promise((res, rej) => {
resolve = res; reject = rej
})
return { promise, resolve, reject }
}
const tasks = [ deferred(), deferred(), deferred() ] // 3 "deferrred" objects
Promise.all(tasks.map(t => t.promise)).then(console.log, console.error)
setTimeout(_ => tasks[0].resolve("one"), 1000)
setTimeout(_ => tasks[1].resolve("two"), 2000)
setTimeout(_ => tasks[2].resolve("three"), 3000)