我可以将新的 promise 推入 Promise.all() 中间的数组吗？

Question

I'm trying to figure out how Promise.all works. My question is: is that ok to push a new promise in the middle of Promise.all() into the same array that was passed earlier? For example, I have this code which works just fine: https://jsfiddle.net/artheg/z0Lndw1o/14/

const promises = [];
const promise1 = new Promise(function(resolve, reject) {
  resolve('promise1');
  const promise2 = new Promise(function(resolve, reject) {
    resolve('promise2');
  });
  pushPromise(promise2); // Pushing promise2 to the same array after the promise1 was resolved
});

pushPromise(promise1); // Pushing promise1

function pushPromise(promise) {
  promises.push(promise);
}

Promise.all(promises).then(function(values) {
  console.log(values); // Array [ "promise2", "promise1" ]
});

Is this behavior intended or should I always chain promises with.then() instead of pushing a new Promise into the array?

Answer 1

That's because you push the promises to the array synchronously . Promise.all works with the promises that are in the array by the time you pass it in.

Example of a Promise added asynchronously:

 const promises = [Promise.resolve(1)];

 setTimeout(() => promises.push(Promise.resolve(2)), 0);

 Promise.all(promises).then(console.log); // [1]

Answer 2

The code in a promise executor (the function you pass new Promise ) runs synchronously . So pushPromise(promise2) has already run before you call Promise.all . The array is already filled by the time you call Promise.all .

Can I push new promise into array in the middle of Promise.all()?

Not if you want Promise.all to see that promise, no. See the specification , Promise.all loops through the iterable you pass it synchronously, before returning, and before any handlers it ( Promise.all ) sets up on the promises can be called.

Here's an example, note that the promise from Promise.all is fulfilled rather than rejecting, even though the code adds a promise to the array that it rejects:

 const promises = [ Promise.resolve(1), Promise.resolve(2).finally(() => { promises.push(Promise.reject(new Error("failed"))); }), Promise.resolve(3), ]; Promise.all(promises).then(results => { console.log(`results (${results.length}): ${results.join(", ")}`); console.log(`promises.length: ${promises.length}`); }).catch(error => { console.error(error); });