CODEWITHSUNDEEP
javascriptpromiseecmascript-6
Li Hang
Li Hang

Reputation: 95

Executing promises in sequence: understanding a claim made about this example code

I'm not looking for a solution for how to execute promises in sequence, I'm trying to understand the one given by a blog author:

I am reading "We have a problem with promises" by Nolan Lawson, posted 18 May 2015.

Under "Advanced mistake #3: promises vs promise factories" he has this example that is supposed to be the working one:

// Setup, just a meta explanation what a function looks like
function promiseFactory() {
  return somethingThatCreatesAPromise();
}

// Actual example; promiseFactories could be an Array
function executeSequentially(promiseFactories) {
    var result = Promise.resolve();
    promiseFactories.forEach(function (promiseFactory) {
        result = result.then(promiseFactory);
    });
    return result;
}

I don't understand why that works. forEach executes all functions at once. result will be a resolved promise so promiseFactory is run right away?

I don't get how this code is supposed to ensure the code in the promiseFactory functions runs in sequence.

Upvotes: 2

Views: 96

Answers (1)

CodingIntrigue
CodingIntrigue

Reputation: 78545

forEach executes all functions at once

No, it runs them one after another. forEach is not asynchronous.

So let's look at an example iteration:

// The first result is already resolved, so as to allow
// the first 'then' to trigger
var result = Promise.resolve();
promiseFactories.forEach(function (promiseFactory) {
    // First iteration: result = ResolvedPromise.then(promiseFactory)
    // Second iteration result = promiseFactoryResult.then(nextPromiseFactory);
    // and so on...
    result = result.then(promiseFactory);
});

So it loops through the array-like object, one after another, chaining the result of promises created by the nextPromiseFactory onto the result variable.

It's essentially the same as doing:

var result = Promise.resolve().then(promiseFactory).then(promiseFactory).then(promiseFactory);

Upvotes: 3

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