Get value of resolved Promise in sync

问题:

If we know that a Promise is definitely resolved, how can we access the value and if we can't, why not?

let a = Promise.resolve(123);

console.log(a.value); // ???

The following does not work- it prints "First, Last, 123"

console.log("First");
Promise.resolve(123).then(console.log);
console.log("Last");

I'm asking how to get the value of an already resolved Promise synchronously and if that's not possible, why not?

回答1:

No, it is not possible to do this. This is by design.

The Promise A+ specification is meant to be used as a simple, consistent way to deal with asynchronous operations. One of the constraints is that passing a new callback on to then() on an already resolved promise will always execute on a later tick in the event loop, so things are consistent.

Adding a secondary way to inspect promise results would not have been impossible. It would probably have been quite easy to do so, but there's at least 2 problems with adding this to the specification:

1. If you're looking to build a specification, you want it to be as simple as possible. The specification itself actually only defines the then() function.
2. By adding this feature, you're opening the door to hordes of developers getting even more confused about something that's already hard to grok.

Promises and asynchronous operations are hard for people to understand. I see questions here daily about promises and not 'getting' it. If non-async way to access promise results would be added to the default promise, I'd imagine that this would be an even larger amount. It's good to try and enforce 'the right way' of doing things.

However, this decision is not simply made for you. You're not stuck there. You can very easily build your own version of a promise that has this feature, and still works with existing promises. As long as your object has a then() function that behaves according to Promises/A+ you can do with the rest of the object whatever you want.

回答2:

You can use async/await syntax from ES7

(async () => {
  console.log("First");
  console.log(await Promise.resolve(123));
  console.log("Last");
})()

Remember that await only works inside async functions

Explanation "Why you can't get the value of an already resolved Promise synchronously"

Promise debugging

By design, the instant state and value of a promise cannot be inspected synchronously from code, without calling the then() method.

To help with debugging, only when inspecting a promise object manually, you can see more information as special properties that are inaccessible from code (this, at present, is implemented by randomizing the property name, for the lack of more sophisticated language or debugger support).

回答3:

Promise.resolve(123).then(function(value) {
    console.log(value);
}

Take a look on this

回答4:

Promises are always asynchronous in JS.

If you're confident that promise is going to resolve then you can access it with .then method.

a.then(function(value) {
    console.log("First");
    console.log(value);
    console.log("Last");
    // expected output: 123
});

Variable a will lookalike below if it is console.log

Promise {<resolved>: 123}
    __proto__: Promise
    [[PromiseStatus]]: "resolved"
    [[PromiseValue]]: 123

For error handling, you can error block as mentioned in MDN docs.