Promise.all与x => Promise.all（x）在诺言链中有什么区别？

Question

I've been trying to get an array of promises to resolve all at the same time. I know that Promise.prototype.then() takes a callback which accepts the resolved value:

 const one = Promise.resolve(1) one.then( console.log ) // 1 one.then( x => console.log(x) ) // 1 

When I try to invoke Promise.all on an array of promises, I have to use a callback to make it work.

 const two = Promise.resolve(2) const three = Promise.resolve(3) Promise.resolve([two, three]) .then( xs => Promise.all(xs) ) // Works .then( console.log ) // [2,3] Promise.resolve([two,three]) .then( Promise.all ) // Error .then( console.log ) 

What's going on here? Why can't I just pass in Promise.all and have it work as the callback function? Why do I have to 'manually' call it?

Answer 1

Promise.all expects to be called with Promise * as the this object. This would work (but be more verbose than the arrow function):

Promise.resolve([two, three])
  .then(Promise.all.bind(Promise))
  .then(console.log)

* Technically, this is specified here ; it can be called on another constructor that is similar to Promise in a number of ways, but in practice you'll want to use Promise itself.

Answer 2

I might be missing something but isn't this what you're trying to accomplish:

const one = Promise.resolve(1);
const two = Promise.resolve(2);
const three = Promise.resolve(3);

Promise.all([one,two,three]).then(console.log);