Consuming functions can be registered (subscribed) using methods For instance, here’s a reaction to a successfully resolved promise:If we’re interested only in successful completions, then we can provide only one function argument to If we’re interested only in errors, then we can use And here there’s an error in the promise, passed through We’ll talk more about promise chaining and result-passing between handlers in the next chapter.Note that this is different, and more powerful than the real life “subscription list” scenario. Note that promises are guaranteed to be asynchronous. A Promise object is created using the new keyword and its constructor. The JavaScript promises API will treat anything with a then() method as promise-like (or thenable in promise-speak sigh), so if you use a library that returns a Q promise, that's fine, it'll play nice with the new JavaScript promises. The termination condition of these functions determines the "settled" state of the next promise in the chain. First, we run loadScript(script), and .then we write what to do with the result. Sometimes there is no choice, because an error must be handled immediately. The new promise resolves when all listed promises are settled, and the array of their results becomes its result. And even if something goes very wrong, say, a fire in the studio, so that you can’t publish the song, they will still be notified.Everyone is happy: you, because the people don’t crowd you anymore, and fans, because they won’t miss the single.This is a real-life analogy for things we often have in programming:The analogy isn’t terribly accurate, because JavaScript promises are more complex than a simple subscription list: they have additional features and limitations. If the singer has already released their song and then a person signs up on the subscription list, they probably won’t receive that song. To force more errors, change the The fulfillment of the promise is simply logged, via a fulfill callback set using This example is started by clicking the button. executor 1. (You need a browser that supports By clicking the button several times in a short amount of time, you'll even see the different promises being fulfilled one after another.Get the latest and greatest from MDN delivered straight to your inbox.The newsletter is offered in English only at the moment.

На promise можно навешивать колбэки двух типов: onFulfilled – срабатывают, когда promise в состоянии «выполнен успешно». : We must have a callback function at our disposal when calling loadScript(script, callback).In other words, we must know what to do with the result before loadScript is called.

O executor é chamado antes que o construtor de Promise retorne o objeto criado. But there’s more.

Therefore, an action for an already "settled" promise will occur only after the stack has cleared and a clock-tick has passed. If you haven’t already created an account, you will be prompted to do so after signing in. Although, as I mentioned, jQuery's Deferreds are a bit … unhelpful. (See The signatures of these two functions are simple, they accept a single parameter of any type. Promise.all takes an array of promises (it technically can be any iterable, but is usually an array) and returns a new promise.. To understand this, start by scrolling to the bottom of the code block, and examine the promise chain. For instance, the Promise.all below settles after 3 seconds, and then its result is an array [1, 2, 3]: The chain is composed of This code can be run under NodeJS. You give your fans a list. This feels like a good time to warn you that what jQuery calls a promise is in fact totally different to what everyone else calls a promise. This function should take two functions as parameters.

We immediately have a resolved promise.A Promise object serves as a link between the executor (the “producing code” or “singer”) and the consuming functions (the “fans”), which will receive the result or error. Sign in to enjoy the benefits of an MDN account. : We can call .then on a Promise as many times as we want. onRejected – срабатывают, когда promise в состоянии «выполнен с … Promises Callbacks; Promises allow us to do things in the natural order. p Promises are useful both in node.js and the browser. The effect is much like that of This example shows diverse techniques for using Promise capabilities, and diverse situations that can occur. Comprehension is enhanced by seeing the errors actually occur. To learn about the way promises work and how you can use them, we advise you to read Handling a rejected promise too early has consequences further down the promise chain. When it is finished with the attempt it calls Later we’ll see how “fans” can subscribe to these changes.Here’s an example of a promise constructor and a simple executor function with “producing code” that takes time (via The executor is called automatically and immediately (by After one second of “processing” the executor calls That was an example of a successful job completion, a “fulfilled promise”.And now an example of the executor rejecting the promise with an error:To summarize, the executor should perform a job (usually something that takes time) and then call A promise that is either resolved or rejected is called “settled”, as opposed to an initially “pending” promise.The idea is that a job done by the executor may have only one result or an error.In case something goes wrong, the executor should call In practice, an executor usually does something asynchronously and calls For instance, this might happen when we start to do a job but then see that everything has already been completed and cached.That’s fine. Uma função que recebe dois argumentos resolve e reject. We can add handlers any time: if the result is already there, our handlers get it immediately.Next, let’s see more practical examples of how promises can help us write asynchronous code.Here’s the callback-based variant, just to remind us of it:We can immediately see a few benefits over the callback-based pattern:So promises give us better code flow and flexibility. These functions are written by you, the programmer.


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