Once the actual result 'DONE' of the asynchronous computation is ready, it is delivered via resolve line B , which is a parameter of the callback that starts in line A. The following code invokes asyncFunc from the async function main. Namely, asyncFunc is a blocking function call:. Invoking that callback is the same as resuming the code. If a function returns a Promise then that Promise is like a blank into which the function will usually fill in its result, once it has computed it.
You can simulate a simple version of this process via an Array:. Registering the event listener line B can be done after calling asyncFunc , because the callback handed to setTimeout line A is executed asynchronously after this piece of code is finished. Normal event emitters specialize in delivering multiple events, starting as soon as you register. In contrast, Promises specialize in delivering exactly one value and come with built-in protection against registering too late: the result of a Promise is cached and passed to event listeners that are registered after the Promise was settled.
Once a result was delivered via a Promise, the Promise stays locked in to that result. That means each Promise is always in either one of three mutually exclusive states:. A Promise is settled the computation it represents has finished if it is either fulfilled or rejected. A Promise can only be settled once and then stays settled. Subsequent attempts to settle have no effect. As a consumer of promise , you are notified of a fulfillment or a rejection via reactions — callbacks that you register with the methods then and catch :.
Note that catch is simply a more convenient and recommended alternative to calling then. That is, the following two invocations are equivalent:. A Promise library has complete control over whether results are delivered to Promise reactions synchronously right away or asynchronously after the current continuation, the current piece of code, is finished.
It states so via the following requirement 2.
Promises: The Definitive Guide
Additionally, this constraint prevents you from writing functions that sometimes return results immediately, sometimes asynchronously. This is an anti-pattern, because it makes code unpredictable. The following code is a Promise-based version of the built-in Node.
Note that in line A, we are calling resolve with zero parameters, which is the same as calling resolve undefined. Just being notified is enough here. Note that the rejection after the timeout in line A does not cancel the request, but it does prevent the Promise being fulfilled with its result. Now we are ready to dig deeper into the features of Promises.
That means that you can use Promise. In fact, it is used by Promise. In this section, we take a closer look at how Promises can be chained. The result of the method call:. That means that you can keep the Promise-based control flow going by invoking then on Q:. If you resolve the Promise Q returned by then with a normal value, you can pick up that value via a subsequent then :. You can also resolve the Promise Q returned by then with a thenable R. A thenable is any object that has a method then that works like Promise.
Thus, Promises are thenables. Resolving with R e. In a way, Q becomes R. 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:.
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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.
promises: Abstractions for Promise-Based Asynchronous Programming
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: Given two Promises P and Q, the following code produces a new Promise that executes Q after P is fulfilled. 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 governor promised that the prisoners would receive a fair trial.
The Five Promises
She promised to announce the results tomorrow. International organizations have promised aid. I promise to be careful.
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You always promise , but you never do what you say you will. Recent Examples on the Web: Noun The neons and blues of planet Promethea's urban guerrilla warfare provide a welcome change from the brown and gray desert environments Borderlands is generally known for Pre-Sequel also showed a lot of promise on this score. Johnson writes. First Known Use of promise Noun 15th century, in the meaning defined at sense 1a Verb 15th century, in the meaning defined at transitive sense 1.
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