Can an ES6 JavaScript promise be resolved by anything else if it is not resolved by the executor?

问题

If a promise is created this way:

myPromise = new Promise(function(resolve, reject) {});

Can this promise be ever resolved by any way? Obviously, the executor is not resolving it in this case, so can the promise be ever resolved?

I think if it is jQuery, it can be resolved by deferred.resolve() so that the related promise deferred.promise() is resolved.

回答1:

No, the promise can only be resolved or rejected by the resolve and reject functions, respectively.

If these functions aren't called nor exposed, the promise will remain in pending state forever.

That's also a security level, as it helps you ensure the promise you wait for will resolve to the value you expect and when you expect; and allows you to safely return an internal promise, as it is (without any hazards or leaking secrets).

To make a promise, that relies on another promise you don't have control over, but can be resolved (or rejected) manually, regardless of the original, you can use the Promise.race() function:

//The original promise that may pend forever
const original = new Promise(()=>{})

//Controller promise, whose resolver is exposed (or contains the logic needed for "manual" resolution)
let resolve
const controller = new Promise(rs=>{resolve=rs})

//The promise you wait for (and can be resolved or rejected either by `original` or `controller`)
const mixed = Promise.race([original,controller])

mixed.then(console.log)

setTimeout(()=>resolve('Resolved'), 2000)

回答2:

Assign the resolve function to a variable outside the scope of the promise

回答3:

Above solutions are nice, but I think we can create separate function / class / whatever to make it more hermetic

function CreatePromise (executor) {
    let resolve;
    let reject;

    const promise = new Promise((res, rej) => {
        resolve = res;
        reject = rej;
        executor(res, rej)
    })

    return {
        resolve,
        reject,
        promise
    }
}

const { reject, promise } = CreatePromise((resolve) => {
    setTimeout(() => {
        resolve()
    }, 100000)
})

来源：https://stackoverflow.com/questions/59559897/can-an-es6-javascript-promise-be-resolved-by-anything-else-if-it-is-not-resolved

标签

javascript

promise

es6-promise