Promise chaining: Use result from previous promise

问题:

This question already has answers here:

How do I access previous promise results in a .then() chain? (17 answers)

Closed 5 years ago.

I'm using straight ES6 Promises (with the es6-promise polyfill library) and I'm running into a problem with accessing results from previous promises in chained ones.

This problem is identical in the context of Angular/Q, but I'm dissatisfied with the answer and wanted to see if there's a better way:

How to access result from the previous promise in AngularJS promise chain?

Consider the code snippet below:

Student.find().then(function(student) {
        return HelpRequest.findByStudent(student);
    }, function(error) { //... }
).then(function(helpRequest) {
    // do things with helpRequest...
    // PROBLEM: I still want access to student. How can I get access to it?
});

In the chained promise, I want to use the student object that I got in the first promise. But as written, this can't access it. I have a couple apparent options:

1. store the student in a variable in an outer scope (yuck)

2. I actually don't know how this would work, but the solutions in the other question suggest I can call then on the result of HelpRequest.findByStudent() and Promise.resolve the combined result inside the Student.find().then call. The below implementation won't work I think, though.

   Student.find().then(function(student) {
           var data = {student: student};
           HelpRequest.findByStudent(student).then(function(helpRequest) {
               data.helpRequest = helpRequest;
           });
           // PROBLEM: if HelpRequest.findByStudent(student) is asynchronous, how 
           // does this get the data before returning?
           return data; 
       }, function(error) { //... }
   ).then(function(helpRequest) {
       // do things with helpRequest and student
   });
   

I absolutely don't want to do the processing of the helpRequest nested inside of the Student.find() method, as that defeats the purpose of chaining Promises; and even if the second example can be worked into a usable state, it still feels like a hack.

Is there a better way to do achieve this without having to introduce global state or nesting into my code? For instance, is there a way to call Promise.resolve() on multiple values, some of which may be promises and some of which are not?

I'm curious, hope I have alternatives/can understand how to make this work properly without introducing nesting or state!

回答1:

In my opinion, the zen of promises is all about figuring out they're really just asynchronous values. If you start using them as such these problems become simpler in many cases. It's not a silver bullet but it sure does help:

In ES5:

var student = Student.find();
var helpRequest = student.then(HelpRequest.findByStudent);
Promise.all([student, helpRequest]).then(function(results){
    var student = results[0];
    var helpRequest = results[1];
    // access both here
});

In ES6, with all its features:

var student = Student.find();
var helpRequest = student.then(HelpRequest.findByStudent);
Promise.all([student, helpRequest]).then(([student, helpRequest]) => {
    // access both here
});

In another richer promise library (bluebird):

var student = Student.find();
var helpRequest = student.then(HelpRequest.findByStudent);
Promise.join(student, helpRequest, function(student, helpRequest){
    // access both here
});