Using Promise.resolve().then has no advantages over nextTick. It runs on the same queue, but have slightly higher priority, that is, promise handler can prevent next tick callback from ever running, the opposite is not possible. This behaviour is an implementation detail and should not be relied on. Promise.resolve().then is obviously slower (a lot, I think), … Read more
Just return Promise.all(… getPromise1().then(() => { return Promise.all([getPromise2(), getPromise3()]); }).then((args) => console.log(args)); // result from 2 and 3
Promise.all will reject as soon as one of the Promises in the array rejects. Promise.allSettled will never reject – it will resolve once all Promises in the array have either rejected or resolved. Their resolve values are different as well. Promise.all will resolve to an array of each of the values that the Promises resolve … Read more
Immediately, yes, by specification. From the MDN: The executor function is executed immediately by the Promise implementation, passing resolve and reject functions (the executor is called before the Promise constructor even returns the created object) This is defined in the ECMAScript specification (of course, it’s harder to read…) here (Step 9 as of this edit, showing … Read more
If your question is “why does response.json() return a promise?” then @Bergi provides the clue in comments: “it waits for the body to load”. If your question is “why isn’t response.json an attribute?”, then that would have required fetch to delay returning its response until the body had loaded, which might be OK for some, … Read more
Isn’t there a .always like jQuery has? No, there’s not (yet). Though there is an active proposal, so maybe ES2018. Yes, there is: promise .finally() is part of the standard since ES2018. If not, how do I achieve this? You can implement the finally method yourself like this: Promise.prototype.finally = function(cb) { const res = … Read more
Bergi’s answer is excellent, and has been very helpful to me. This answer is complementary to his. In order to visualize the relationship between the Promise() constructor and the then() method, I created this diagram. I hope it helps somebody… maybe even me, a few months months from now. The main idea here is that … Read more
NO you can’t get the data synchronously out of a promise like you suggest in your example. The data must be used within a callback function. Alternatively in functional programming style the promise data could be map()ed over. If your are OK using async/await (you should it’s awesome) then you can write code that looks … Read more
Just as its name implies, the await keyword will cause the function to “wait” until the corresponding promise resolves before executing the next line. The whole point of await is to provide a way to wait for an asynchronous operation to complete before continuing. The difference between this and blocking code is that the world … Read more
Update: jQuery 3.0 has fixed the problems outlined below. It is truly Promises/A+ compliant. Yes, jQuery promises have serious and inherent problems. That said, since the article was written jQuery made significant efforts to be more Promises/Aplus complaint and they now have a .then method that chains. So even in jQuery returnsPromise().then(a).then(b) for promise returning … Read more