Promise

It is simple and easy to read, but you wouldn't want to use it in most applications as it is blocking. This means that while you are reading the file from disk a slow operation nothing else can happen. To make our application performant and responsive, we need to make all the operations that involve IO be asynchronous. The simplest way to do this would be to use a callback. However, a naive implementation will probably go wrong:.

We need to handle errors thrown by JSON.

Understanding promises in JavaScript

By the time we've done all of this our code is a mess of error handling:. Despite all this mess of error handling code, we are still left with the problem of the extra callback parameter hanging around. Promises help you naturally handle errors, and write cleaner code by not having callback parameters, and without modifying the underlying architecture i. The core idea behind promises is that a promise represents the result of an asynchronous operation.

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A promise is in one of three different states:. In practice, this requirement ensures that onFulfilled and onRejected execute asynchronously, after the event loop turn in which then is called, and with a fresh stack. That is, in strict mode this will be undefined inside of them; in sloppy mode, it will be the global object. Generally, it will only be known that x is a true promise if it comes from the current implementation. This clause allows the use of implementation-specific means to adopt the state of known-conformant promises.

This procedure of first storing a reference to x. Such precautions are important for ensuring consistency in the face of an accessor property, whose value could change between retrievals. Resolving with R e. In a way, Q becomes R.

Promises/A+

The main use for this mechanism is to flatten nested then calls, like in the following example:. Whatever you return in an error handler becomes a fulfillment value not rejection value! That allows you to specify default values that are used in case of failure:. Exceptions that are thrown in the callbacks of then and catch are passed on to the next error handler, as rejections:. Then the error is passed on until there is an error handler. In the following code, a chain of two Promises is built, but only the first part of it is returned. As a consequence, the tail of the chain is lost.

The fix is to un-nest this code by returning the second Promise from the first then and handling it via a second, chained, then:. In the following code, method insertInto creates a new Promise for its result line A:. If you look closely, you can see that the result Promise is mainly used to forward the fulfillment line C and the rejection line D of the asynchronous method call db.

In principle, catch cb is an abbreviation for then null, cb. But using both parameters of then at the same time can cause problems:. The rejection callback line D receives all rejections of asyncFunc1 , but it does not receive rejections created by the fulfillment callback line A. For example, the synchronous function call in line B may throw an exception or the asynchronous function call in line C may produce a rejection. For operational errors, each function should support exactly one way of signaling errors. For programmer errors, it can make sense to fail as quickly as possible, by throwing an exception:.

If you do this, you must make sure that your asynchronous code can handle exceptions. I find throwing exceptions acceptable for assertions and similar things that could, in theory, be checked statically e. If an exception is thrown in line A then the whole function throws an exception.

There are two solutions to this problem. You can also start a chain of then method calls via Promise. This approach saves you a tick the synchronous code is executed right away , but it makes your code less regular. Composing means creating new things out of existing pieces. We have already encountered sequential composition of Promises: Note that this is similar to the semicolon for synchronous code: Sequential composition of the synchronous operations f and g looks as follows. The two function calls asyncFunc1 and asyncFunc2 are made without then chaining. As a consequence, they are both executed immediately and more or less in parallel.

Once both threads are finished with a result or an error , execution is joined into a single thread in either handleSuccess or handleError. The problem with this approach is that it involves too much manual and error-prone work. The fix is to not do this yourself, by relying on the built-in method Promise. Once all of them are fulfilled, it fulfills with an Array of their values. If iterable is empty, the Promise returned by all is fulfilled immediately. One nice thing about Promises is that many synchronous tools still work, because Promise-based functions return results.

For example, you can use the Array method map:.

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We can use Promise. The first of the input Promises that is settled passes its settlement on to the output Promise. If iterable is empty then the Promise returned by race is never settled. This section describes two useful methods for Promises that many Promise libraries provide.

They are only shown to further demonstrate Promises, you should not add them to Promise. If then in line A produces a rejection, it will never be handled anywhere. The Promise library Q provides a method done , to be used as the last element in a chain of method calls. It either replaces the last then and has one to two arguments:. Quoting the Q documentation:. The Golden Rule of done versus then usage is: Terminating with catch is not sufficient because the catch handler may itself throw an error.

The idea was to first explore how much engines can detect automatically. Depending on how well that works, it may to be necessary to introduce done. Sometimes you want to perform an action independently of whether an error happened or not.

promise - Wiktionary

For example, to clean up after you are done with a resource. Its callback receives no arguments, but is notified of either a resolution or a rejection. This is how Domenic Denicola proposes to implement finally:. Example 1 by Jake Archibald: Example 2 by Kris Kowal: The Promise library Q has tool functions for interfacing with Node.

For example, denodeify converts a callback-based function to a Promise-based one:. There are many Promise libraries out there. The solution is to bring blocking calls to JavaScript. Generators let us do that, via libraries: