Promises.all() tracking invidual tasks completion

Question

I'm using Promise.all(tasks) to track the overall completion of my tasks executed in no particular order (which is my main aim). I also want to track individual tasks completion, how do I do that?

What do you mean, individual? One of the tasks completed? Or exact task completed? — Dmitry Yaremenko, Jan 27 '16 at 15:22
*track* - and do what with the information (which information exactly)? — Bergi, Jan 27 '16 at 15:22
Just install individual `then` handlers on each of the promises? — Bergi, Jan 27 '16 at 15:23
Use `[/* array of promises */].map()` as parameter to `Promise.all()` ? "track" each `Promise` within `.map()` callback ? — guest271314, Jan 27 '16 at 15:24
@Bergi then will force my tasks to run sequentially... I don't want TaskA to wait till TaskB completes, you are missing the point. — user1514042, Jan 27 '16 at 15:28
@user1514042: `then` doesn't force any sequence, unless you're creating TaskB in the callback of TaskA - which is not what I was referring to. — Bergi, Jan 27 '16 at 15:30

score 1 · Accepted Answer · answered Jan 27 '16 at 15:28

1

Nothing keeps you from attaching individual handlers as well awaiting them together:

let tasks = …;
for ([t, i] of tasks.entries())
    t.then(res => {
        console.log("task "+i+" completed with", res);
    }, err => {
        console.log("task "+i+" failed because", err);
    });
Promise.all(tasks).then(all => {
    console.log("all tasks completed");
}, err => {
    console.log("something failed");
});

answered Jan 27 '16 at 15:28

Bergi

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would then() not force sequential execution? – user1514042 Jan 27 '16 at 15:29
Yes, it would force the `console.log`s to run after the respective promise settles. It doesn't force any order between the promises. Just try it on your `tasks`! – Bergi Jan 27 '16 at 15:30

score 1 · Answer 2 · answered Jan 27 '16 at 15:36

1

Try using Array.prototype.map() to pass promises to Promise.all()

var promises = [Promise.resolve("a"), Promise.resolve("b")];

Promise.all(promises.map(function(p, index) {
  return p.then(function(data) {
    console.log("inside .map()", data, "index", index)
    return data
  }, function(err) {
    console.log(err);
    return err
  })
}))
.then(function(complete) {
  console.log("all promises after .map()", complete)
}, function(err) {
  console.log("err", err)
})

answered Jan 27 '16 at 15:36

guest271314

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That'll hardly ever hit `console.log("err", err)` early-reject, as you're handling all errors – Bergi Jan 27 '16 at 15:38
@Bergi Can describe "hardly ever hit console.log("err", err)" ? Will not reach `onRejected` of `.then` following `.all()` ? – guest271314 Jan 27 '16 at 15:41
Yes, it will not reach it when any of the `promises` is rejected. – Bergi Jan 27 '16 at 15:42
@Bergi Interestingly just tried with second promise converted to `.reject("b")` , `"all promises after .map() ["a", "b"]"` logged at `.then` `onFulfilled` https://jsfiddle.net/9gprLc7q/ ? Would expect expect `console.log("err", err)` to be called ? Any insight ? – guest271314 Jan 27 '16 at 15:45
1

@Bergi http://stackoverflow.com/questions/35042068/why-is-onrejected-not-called-following-promise-all-where-promise-reject-incl – guest271314 Jan 27 '16 at 15:56

Promises.all() tracking invidual tasks completion

2 Answers2