如何异步编程（承诺）JavaScript实现？是不是JavaScript的一个UI线程环境中？

问题描述

限时删除！！

在JS承诺允许你这样做异步编程，如下：

  DoSomething的（），然后（成功，失败）。DoSomethingElse（）;
 

每当我写的previous code达到 DoSomethingElse（）到达之前成功 。
这怎么可能？是不是JS单线程环境（不包括网络工作人员）？它是用的setTimeout 做了什么？

解决方案

是，JavaScript是单线程的，这意味着你永远不应该阻止该单个线程。任何长期运行，等待操作（通常是AJAX调用或休眠/暂停）使用回调来实现。

不看这里的实现是会发生什么：

1. 的DoSomething 被调用，它接收成功和失败函数作为参数。




2. 它做什么它需要做的（可能引发长期运行的AJAX调用），并返回




3. DoSomethingElse（）被称为




4. ...




5. 一段时间后AJAX响应到达。它调用previously定义成功和失败函数

参见（类似问题）

Promises in JS allow you to do async programming, as follows:

DoSomething().then(success, failure);

DoSomethingElse();

whenever i write the previous code it reaches DoSomethingElse() before it reaches success.How is that possible? Isn't JS a single threaded environment (not including web-workers)? is it done with setTimeout?

解决方案

Yes, JavaScript is single-threaded, which means you should never block this single thread. Any long-running, waiting operation (typically AJAX calls or sleeps/pauses) are implemented using callbacks.

Without looking at the implementation here is what happens:

1. DoSomething is called and it receives success and failure functions as arguments.

2. It does what it needs to do (probably initiating long-running AJAX call) and returns

3. DoSomethingElse() is called

4. ...

5. Some time later AJAX response arrives. It calls previously defined success and failure function

See also (similar problems)

• JavaScript equivalent of SwingUtilities.invokeLater()
• Are there any atomic javascript operations to deal with Ajax's asynchronous nature?
• jqGrid custom edit rule function using Ajax displays "Custom Function should return array!"

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