q库承诺向完成的方法传递一个值

Question

I'm trying to find the purpose as well as how to use done method of the q library promises, if done can receive a value or function via resolve or reject , can someone explain how done method is called and how can pass any arguments to it?

Q.fcall(promisedStep1)
.then(promisedStep2)
.then(promisedStep3)
.then(promisedStep4)
.then(function (value4) {
    // Do something with value4
})
.catch(function (error) {
    // Handle any error from all above steps
})
.done();

Answer 1

In promises, whatever you resolve your deferred with, is the argument that get's passed to done or then . You can change the resolve value inside of a resolved handler by returning a different value. Like so

Q.fcall(promisedStep1)
.then(promisedStep2)
.then(promisedStep3)
.then(promisedStep4)
.then(function (value4) {
  // Do something with value4
  return 'tada!';
})
.catch(function (error) {
    // Handle any error from all above steps
})
.done(function(differentValue) {
  console.log(differentValue); // outputs "tada!"
});

Answer 2

The actual purpose of .done in Q is to handle errors so errors won't get suppressed.

If you have a rejected promise chain in Q, if you use .then all the way it becomes a silent failure, so the following code:

Q().then(function(){
    var val = JSON.prase(data);
    someEffectWith(val);
});

Is a silent failure, did you note the typo? Because there is never a way to know when the chain has ended it is crucial to use .done to let the library know:

Q().done(function(){
    var val = JSON.prase(data);
    someEffectWith(val);
});

Or:

Q().then(function(){
    var val = JSON.prase(data);
    someEffectWith(val);
}).done();

Will both echo a big red warning to your console, notifying you of the error. As for how it's called it has the exact same arguments of .then only it does not return a promise, instead it returns undefined so you know you can't chain to it (it terminated the chain).

It's worth mentioning some promise libraries, as well as native promises in firefox do this for you and you don't need to use .done in those libraries - the error will get logged regardless.