JavaScript 承诺中的隐式“解决”

Question

I'm using promises in JavaScript with setTimeout and I'm getting something of a mysterious behaviour. I'm seeing things working fine and I can't explain it.

Let's say I have this function:

function f(callback) {
  setTimeout(() => callback(13), 3000);
}

When I call it with no callback, I get a most expected error:

> f()
undefined
> Uncaught TypeError: callback is not a function

When I call it with an actual callback, all works fine:

> f(console.log)
undefined
> 13

The «problem»

So far, all went according to plan, but when I call f() wrapped in a promise, it seems to no longer require the callback parameter:

> new Promise(f).then(console.log)
Promise { <pending> }
> 13

Going a bit slower, if I store the promise, it ends up having a value:

> p = new Promise(f)
Promise { <pending> }
> p
Promise { <pending> }
> p // after 3 seconds
Promise { 13 }
>

This happens on both browser and Node.

QUESTION: How is it possible that f doesn't complain at all for the lack of a callback ? How is the value 13 assigned as the promise value?

TL;DR === false I know I should be counting my blessings because it's working, but documentation doesn't seem to mention any hidden default mechanism for dealing with this kind of situation that I could find. All I get is a brief intro immediately followed by a fast-forward pathway into chaining and more advanced patterns. I need to be sure that I can replicate this behaviour with more complex cases.

Answer 1

How is it possible that f doesn't complain at all for the lack of a callback?

The function you give to a promise does receive arguments: resolve and reject callbacks.

A normal use of promises look like this:

return new Promise((resolve, reject) => {
  // call resolve() on success
  // call reject() on error
});

How is the value 13 assigned as the promise value?

You're calling resolve(13) when your timeout expires.

Answer 2

Lets move f to the promise constructor...

new Promise(function f(callback) {
  setTimeout(() => callback(13), 3000);
}).then(console.log)

Then lets add another argument...

new Promise(function f(callback, reject) {
  setTimeout(() => callback(13), 3000);
}).then(console.log)

And rename callback to resolve:

new Promise(function f(resolve, reject) {
  setTimeout(() => resolve(13), 3000);
}).then(console.log)

Does it look familiar?