Javascript 什么是闭包和回调？
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What are Closures and Callbacks?
提问by AnthonyWJones
What are closures and callbacks in JavaScript? I've yet to find a good explanation of either.
JavaScript 中的闭包和回调是什么？我还没有找到一个很好的解释。
回答by AnthonyWJones
Closureshave already been well handled in Stackoverflow here is just a selection:-
闭包已经在 Stackoverflow 中得到了很好的处理，这里只是一个选择：-
How does a javascript closure work?
What exactly does “closure” refer to in JavaScript?
can you say this is a right example of Javascript Closure.. Where the places we need to consider avoiding the closures??
JavaScript scope and closure
Javascript Closures and ‘this' context
JavaScript - How do I learn about “closures” usage?
javascript 闭包是如何工作的？
JavaScript 中的“闭包”究竟指的是什么？
你能说这是 Javascript Closure 的一个正确例子.. 我们需要考虑避免闭包的地方在哪里？？
JavaScript 范围和闭包
Javascript 闭包和“this”上下文
JavaScript - 我如何了解“闭包”的用法？
Callbacksare a simpler concept. A callback is basically where a function accepts another function as a parameter. At some point during execution the called function will execute the function passed as a parameter, this is a callback. Quite often the callback actually happens as an asynchronous event, in which case the called function may return without having executed the callback, that may happen later. Here is a common (browser based) example:-
回调是一个更简单的概念。回调基本上是一个函数接受另一个函数作为参数的地方。在执行过程中的某个时刻，被调用的函数将执行作为参数传递的函数，这是一个回调。通常回调实际上是作为一个异步事件发生的，在这种情况下，被调用的函数可能会在没有执行回调的情况下返回，这可能会在稍后发生。这是一个常见的（基于浏览器的）示例：-
function fn() { alert("Hello, World"); }
window.setTimeout(fn, 5000);
Here the function fnis passed as a callbackto the setTimeoutfunction. Set timeout returns immediately however 5 seconds later the function passed as a callbackis executed.
这里函数fn作为回调传递给setTimeout函数。设置超时立即返回，但 5 秒后作为回调传递的函数被执行。
Closures and callbacks
关闭和回调
Quite often the reason that closures get created (either incidentally, accidentally or deliberately) is the need to create a callback. For example:-
很多时候创建闭包的原因（偶然、意外或故意）是需要创建回调。例如：-
function AlertThisLater(message, timeout)
{
function fn() { alert(message); }
window.setTimeout(fn, timeout);
}
AlertThisLater("Hello, World!", 5000);
(Please read some of the linked posts to grasp closures)
（请阅读一些链接的帖子以掌握关闭情况）
A closure is created containing in part the messageparameter, fnis executed quite some time after the call to AlertThisLaterhas returned, yet fnstill has access to the original content of message.
创建一个包含部分message参数的闭包，fn在调用AlertThisLater返回后执行一段时间，但fn仍然可以访问message.
