异步JavaScript：构建更响应式的应用

Async

Javascript

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"Async JavaScript - 建立更响应式应用的无阻塞编程指南" 在JavaScript编程领域，异步编程是一种至关重要的技术，尤其在处理并发任务时，它能够帮助开发者避免程序阻塞，提高应用程序的响应速度。标题"Async JavaScript"直指这一关键主题，而描述中提到的“事件驱动的JavaScript”以及异步如何为世界带来益处，进一步强调了异步编程在现代Web开发中的重要性。异步JavaScript主要关注的是非阻塞I/O操作，它允许代码在等待某些操作（如网络请求或文件读取）完成的同时继续执行其他任务，而不是等待这些操作完成后再进行。这种模式在Web开发中特别有用，因为它使得网页在等待数据加载时仍能保持交互性，提升用户体验。书中作者Trevor Burnham深入浅出地介绍了如何编写异步JavaScript，同时提供了浏览器和服务器端的实例，以帮助读者理解并掌握异步编程的实践技巧。这包括使用回调函数、Promise、async/await等不同的异步处理方法，以及如何处理回调地狱和错误处理等问题。标签"Async Javascript"暗示了这本书的核心内容，即对异步编程概念的全面探讨。书中的内容可能涵盖： 1. **事件循环和事件队列**：讲解JavaScript如何通过事件循环来调度异步任务，理解事件驱动模型的基础。 2. **回调函数**：作为早期异步处理的主要方式，如何正确地编写和管理回调函数，以及如何避免回调地狱。 3. **Promise**：介绍Promise对象，它是解决回调问题的一个重要工具，如何创建、链式调用Promise以及处理错误。 4. **async/await**：讲解ES6引入的异步控制流新特性，如何使用async函数来使异步代码更加清晰、易于理解。 5. **并发与竞争条件**：处理多个异步任务时可能出现的问题，以及如何确保任务的正确执行顺序。 6. **错误处理**：在异步环境中有效地捕获和处理错误，确保程序的健壮性。 7. **性能优化**：如何利用异步编程提高应用程序的性能，减少延迟。 8. **模块化和组织代码**：在异步环境下如何组织代码，使其可维护性和可读性更强。 "Async JavaScript"是一本面向进阶JavaScript开发者的书籍，它将帮助读者掌握异步编程的核心理念和技术，从而构建更响应、结构良好且效率更高的JavaScript应用。对于那些想要提升自己在JavaScript并发编程能力的开发者来说，这本书无疑是一本必读之作。

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But when I started writing the book you’re reading now, I decided that doing

it in CoffeeScript would needlessly limit its appeal. By and large, Coffee-

Scripters understand JavaScript perfectly well, whereas code like

square = (x)

=> x * x

might as well be hieroglyphics to JavaScript purists.

So, if you’re a CoffeeScripter, my apologies for the curly braces. Rest assured

that the lessons you draw from this book will carry over to any altJS language.

Resources for This Book

This book has a website at

http://pragprog.com/book/tbajs/async-javascript

. There you

can download the example code used in the book, get up-to-date information,

and ask book-related questions in a friendly forum.

For more general JavaScript-related questions, I (again) heartily recommend

Stack Overflow.

I have no affiliation with the site, but I am an avid fan with

a proud 23,000 reputation points (and counting). Coherent, well-formatted

questions there are almost always answered promptly.

Finally, if you want to contact me directly, you can reach me at

trevorburn-

ham@gmail.com

or on Twitter:

@trevorburnham

. I’m always happy to hear from my

readers.

Enough introduction. Let’s get our async on!

14. However, maybe not for long:

http://wiki.ecmascript.org/doku.php?id=harmony:arrow_function_syntax

15.

http://stackoverflow.com/

report erratum • discuss

Resources for This Book • xvii

Download from Wow! eBook <www.wowebook.com>

In my multithreaded mind-set, I’d expected only 500ms to go by before the

timed function ran. But that would have required the loop, designed to last

a full second, to be interrupted. Instead, if you run the code, you’ll get

something like this:

Time elapsed: 1002ms

❮

You’ll probably get a slightly different number;

setTimeout

and

setInterval

are,

alas, a lot less precise than you’d hope (see Timing Functions, on page 5).

But it will definitely be at least

1000

, because the

setTimeout

callback can’t fire

until the

while

loop has finished running.

So, if

setTimeout

isn’t using another thread, then what is it doing?

Meet the Queue

When we call

setTimeout

, a timeout event is queued. Then execution continues:

the line after the

setTimeout

call runs, and then the line after that, and so on,

until there are no lines left. Only then does the JavaScript virtual machine

ask, “What’s on the queue?”

If there’s at least one event on the queue that’s eligible to “fire” (like a 500ms

timeout that was set 1000ms ago), the VM will pick one and call its handler

(e.g., the function we passed in to

setTimeout

). When the handler returns, we

go back to the queue.

Input events work the same way: when a user clicks a DOM element with a

click handler attached, a click event is queued. But the handler won’t be

executed until all currently running code has finished (and, potentially, until

after other events have had their turn). That’s why web pages that use Java-

Script imprudently tend to become unresponsive.

You might sometimes hear the term event loop used to describe how the queue

works. It’s as if your code is being run from a loop that looks like this:

runYourScript();

while (atLeastOneEventIsQueued) {

fireNextQueuedEvent();

};

One implication of this is that each event that fires will be at the root of the

stack trace. We’ll learn more about that in Section 1.4, Handling Async Errors,

on page 12.

The ease of event scheduling in JavaScript is one of the language’s most

powerful features. Async functions like

setTimeout

make delayed execution

simple, without spawning threads. JavaScript code can never be interrupted,

report erratum • discuss

Scheduling Events • 3

Download from Wow! eBook <www.wowebook.com>

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