The fact that nests can talk only to their neighbors greatly inhibits the usefulness of this network. It’ll always return only a single line of output, listing the nest that was slowest to respond. We can now send a message to the nest in the church tower, which is four network hops removed. If we had used the handler’s return value as the response value, that would mean that a request handler can’t itself perform asynchronous actions. As programmers we all face it one day or another: asynchronous programming (A.K.A. Async.js is a popular library that has many useful tools for dealing with asynchronous code. Especially when the network is big, that would lead to a lot of useless data transfers. If we have a route toward a message’s destination, we know which direction to send it in. JavaScript is synchronous and single-threaded. In the case of asynchronous actions, you could, instead of arranging for a function to be called at some point in the future, return an object that represents this future event. In terms of what they can express, callbacks and promises are equivalent. When there’s nothing to be done, that loop is stopped. Asynchronous programming - as we know now in JavaScript - can only be achieved with functions being first-class citizens of the language: they can be passed around like any other variables to other functions. Normally, a given program's code runs straight along, with only one thing happening at once. In the current consumer computers, every program runs for a specific time slot, and then it stops its execution to let another program continue its execution. And, since we’ve established that promises are a good thing, we’ll also make our request function return a promise. The difficulty with that is it requires knowledge about the layout of the network. 9 hours, 38 minutes CC. Asynchronous programming and JavaScript Time：2020-7-23 from Promise In the beginning, JavaScript was introducing new features to help make it easier to handle asynchronous programming, and to keep us away from callback hell. As a shorthand, then also accepts a rejection handler as a second argument, so you can install both types of handlers in a single method call. So while we’re at it, let’s make our request function automatically retry the sending of the request a few times before it gives up. This means that code which is is taking some time to finish (like accessing an API, reading content from the local file system etc.) Calling a callback is somewhat more involved and error-prone than simply returning a value, so needing to structure large parts of your program that way is not great. Every time you call next on the iterator, the function runs until it hits a yield expression, which pauses it and causes the yielded value to become the next value produced by the iterator. So with asynchronous JavaScript, the JavaScript doesn’t wait for responses when executing a function, instead it continues with executing other functions. This means that code cannot create new threads and run in parallel. It has a keys method, but that returns an iterator rather than an array. Next, write the same function again without using async and await. JavaScript — Dynamic client-side scripting, General asynchronous programming concepts, Cooperative asynchronous JavaScript: Timeouts and intervals, Handling async operations gracefully with Promises, Making asynchronous programming easier with async and await, Assessment: Structuring a page of content, From object to iframe — other embedding technologies, HTML Table advanced features and accessibility, Assessment: Typesetting a community school homepage, What went wrong? But instead of returning the whole route, it just returns the next step. Asynchronicity cuts both ways. The total time taken will be at least the sum of the two response times. A network request may fail, or some code that is part of the asynchronous computation may throw an exception. An event loop schedules such callbacks to be called when appropriate, one after the other, so that their execution does not overlap. They can use tools, plan ahead, remember things, and even communicate these things among themselves. In the 9th version, ECMAScript 2018, an asynchronous iterator was introduced. Since each nest knows only about its direct neighbors, it doesn’t have the information it needs to compute a route. When one of them succeeds, two things need to happen. It expects the name of the target nest, the type of the request, and the content of the request as its first three arguments, and it expects a function to call when a response comes in as its fourth and last argument. The easiest way to do this is to have that function return a promise by calling then on the promise that retrieves the storage value. In the synchronous model, the time taken by the network is part of the timeline for a given thread of control. This findRoute function, which greatly resembles the findRoute from Chapter 7, searches for a way to reach a given node in the network. So network programming is typically very much about anticipating and dealing with failures. In this chapter we’ll write some basic networking functions for them. It is able to notify anyone who is interested when its value is available. For asynchronous programs this is different—they may have gaps in their execution during which other code can run. That way, when a hawk destroys a nest, the information isn’t lost. It produces a promise when called, which is resolved when it returns (finishes) and rejected when it throws an exception. 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