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Imagine a function, createAudioFileAsync , which asynchronously generates a sound file given a configuration record and two callback functions, one called if the audio file is successfully created, and the other called if an error occurs. If createAudioFileAsync were rewritten to return a promise, using it could be as simple as this:. We call this an asynchronous function call. This convention has several advantages.

  1. Terminology.
  2. U.S. Energy Tax Policy.
  3. Chaining Promises.
  4. Twins Born Apart.
  5. Lexiconc Search.

We will explore each one. A common need is to execute two or more asynchronous operations back to back, where each subsequent operation starts when the previous operation succeeds, with the result from the previous step.

We accomplish this by creating a promise chain. Here's the magic: the then function returns a new promise , different from the original:. This second promise promise2 represents the completion not just of doSomething , but also of the successCallback or failureCallback you passed in, which can be other asynchronous functions returning a promise.

When that's the case, any callbacks added to promise2 get queued behind the promise returned by either successCallback or failureCallback. In the old days, doing several asynchronous operations in a row would lead to the classic callback pyramid of doom:. With modern functions, we attach our callbacks to the returned promises instead, forming a promise chain:. The arguments to then are optional, and catch failureCallback is short for then null, failureCallback. You might see this expressed with arrow functions instead:.

It's possible to chain after a failure, i. Read the following example:. You might recall seeing failureCallback three times in the pyramid of doom earlier, compared to only once at the end of the promise chain:. Basically, a promise chain stops if there's an exception, looking down the chain for catch handlers instead.

This is very much modeled after how synchronous code works:. It builds on promises, e.

You can read more about the syntax here. Promises solve a fundamental flaw with the callback pyramid of doom, by catching all errors, even thrown exceptions and programming errors.


This is essential for functional composition of asynchronous operations. Whenever a promise is rejected, one of two events is sent to the global scope generally, this is either the window or, if being used in a web worker, it's the Worker or other worker-based interface. The two events are:. In both cases, the event of type PromiseRejectionEvent has as members a promise property indicating the promise that was rejected, and a reason property that provides the reason given for the promise to be rejected.

These make it possible to offer fallback error handling for promises, as well as to help debug issues with your promise management. These handlers are global per context, so all errors will go to the same event handlers, regardless of source. One case of special usefulness: when writing code for Node.

Domenic Denicola proof read the first draft of this article and graded me "F" for terminology. He put me in detention, forced me to copy out States and Fates times, and wrote a worried letter to my parents. Despite that, I still get a lot of the terminology mixed up, but here are the basics:. The spec also uses the term thenable to describe an object that is promise-like, in that it has a then method.

If you're a jQuery user, they have something similar called Deferreds. Although promise implementations follow a standardized behaviour, their overall APIs differ. Here's how you create a promise:. The promise constructor takes one argument, a callback with two parameters, resolve and reject. Do something within the callback, perhaps async, then call resolve if everything worked, otherwise call reject.

Promises: The Definitive Guide

Like throw in plain old JavaScript, it's customary, but not required, to reject with an Error object. The benefit of Error objects is they capture a stack trace, making debugging tools more helpful. Both are optional, so you can add a callback for the success or failure case only. To bring browsers that lack a complete promises implementation up to spec compliance, or add promises to other browsers and Node. 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.

Although, as I mentioned, jQuery's Deferreds are a bit … unhelpful. Thankfully you can cast them to standard promises, which is worth doing as soon as possible:. Since it has a then method, Promise. However, sometimes deferreds pass multiple arguments to their callbacks, for example:.

Thankfully this is usually what you want, or at least gives you access to what you want. Also, be aware that jQuery doesn't follow the convention of passing Error objects into rejections. We'll want to stop the spinner at that point too, else it'll keep on spinning, get dizzy, and crash into some other UI. Of course, you wouldn't use JavaScript to deliver a story, serving as HTML is faster , but this pattern is pretty common when dealing with APIs: Multiple data fetches, then do something when it's all done.

Old APIs will be updated to use promises, if it's possible in a backwards compatible way. The response is JSON, but we're currently receiving it as plain text. We could alter our get function to use the JSON responseType , but we could also solve it in promises land:. Since JSON. When you return something from a then callback, it's a bit magic.

If you return a value, the next then is called with that value.


For example:. Here we make an async request to story. This is when promises really start to stand out from simple callback patterns. We don't download story. Yay Promises! As we saw earlier, then takes two arguments, one for success, one for failure or fulfill and reject, in promises-speak :. There's nothing special about catch , it's just sugar for then undefined, func , but it's more readable. Note that the two code examples above do not behave the same, the latter is equivalent to:.

Writing your own promise

The difference is subtle, but extremely useful. Promise rejections skip forward to the next then with a rejection callback or catch , since it's equivalent. With then func1, func2 , func1 or func2 will be called, never both.

PROMISE | meaning in the Cambridge English Dictionary

But with then func1. Take the following:. Here's the above as a flowchart because I love flowcharts :. Rejections happen when a promise is explicitly rejected, but also implicitly if an error is thrown in the constructor callback:. This means it's useful to do all your promise-related work inside the promise constructor callback, so errors are automatically caught and become rejections.

Node.js v10.16.3 Documentation

If fetching story. Instead it moves onto the catch callback. As a result, "Failed to show chapter" will be added to the page if any of the previous actions failed. The above becomes a non-blocking async version of:. You may want to catch simply for logging purposes, without recovering from the error.

To do this, just rethrow the error. We could do this in our getJSON method:. Thinking async isn't easy. If you're struggling to get off the mark, try writing the code as if it were synchronous. In this case:. Try it. That works see code! But it's sync and locks up the browser while things download. To make this work async we use then to make things happen one after another.

But how can we loop through the chapter urls and fetch them in order? This doesn't work :.