Node.js-循环中的承诺

Question

I have the this problem to handle nested promises and loops:

firstPromise().then(function(resultOfFirstPromise){
    var promiseArray = [];
    for(var i = 0; i < 10; i++){
      var ret = secondoPromise();
      promiseArray.push(ret);
    }
    return Promise.all(promiseArray);
}, function(error){
    console.log("firstPromiseError");
}).then(function(resultOfSecondPromise){
    console.log(i); //but obviously i = 10
}, function(error){
    console.log("secondPromiseError");
});

What I want to do is to have the loop index value i in every .then of each secondPromise: the output of my code should by somthing like this:
0 1 2 3 4 5 6 7 8 9 10.

Answer 1

What you are trying to achieve is not possible with Promise . In the chain of .then() , one .then() is run only after all of its previous .then() are done. Meaning, if a .then() has finished running, it will never run again.

I don't know your use case but what you are trying to do is achievable using Rxjs/Observables

let arr = [1, 2, 3, 4, 5];

    Rx.Observable.from(arr)
        .do((v) => {
            /*do some thing*/
            console.log(v)
        })
        .do((v) => {
            /*do another things*/
            console.log(v)
        });

In above code callback inside each do() will run for each element of the arr .

Output :

 1
 1
 2
 2
 3
 3
 4
 4
 5
 5 

Answer 2

Just chain the then calls to secondoRomise() , and also use let in the loop:

firstPromise().then(function(resultOfFirstPromise){
   var promiseArray = [];
   for(let i = 0; i < 10; i++){
     var ret = secondoPromise();
     promiseArray.push(ret);
     ret.then(result => console.log("Promise No." + i + " returned " + result);
   }
   return Promise.all(promiseArray);
}).then(() => console.log("all done!"));

Answer 3

This is just a scope problem because when you create a new function, it creates a new scope so you can't access one scope from another, it acts sort of like creating a new class in Javascript. This is the exact same reason when you create a function you often have to create a variable to reference this (eg const self = this). You can get around this using the ES6 arrow functions, since it doesn't create a new class/scope when you use them.

firstPromise().then( (resultOfFirstPromise) => {
    var promiseArray = [];
    for(var i = 0; i < 10; i++){
      var ret = secondoPromise();
      promiseArray.push(ret);
    }
    return Promise.all(promiseArray);
}, (error) => {
    console.log("firstPromiseError");
}).then( (resultOfSecondPromise) => {
    console.log(i); //but obviously i = 10
}, (error) => {
    console.log("secondPromiseError");
});

Something like that, you may need to finagle a little bit.