Javascript关闭 [英] Javascript closure

问题描述

我在闭包结束时读取（）会立即执行它。那么，这两者之间有什么区别。我在一些代码中看到了第一次使用。

I read the () at the end of the closure will execute it immediately. So, what is the difference between these two. I saw the first usage in some code.

谢谢。

for (var a=selectsomeobj(),i=0,len=a.length;i<len;++i){
        (function(val){
            anotherFn(val);
        })(a[i]);
}

for (var a=selectsomeobj(),i=0,len=a.length;i<len;++i){
            anotherFn(a[i]);
}

推荐答案

在这个例子中没有差异。在这两种情况下， anotherFn 立即执行。

In this example there are no differences. In both cases, anotherFn gets executed immediately.

但是，创建函数时通常会使用立即函数循环。

However, an immediate function is often used when a function is created in a loop.

考虑这个例子（或多或少的伪代码）：

Consider this example (more or less pseudo code):

for(var i from 1..10) {
    elements[i].onclick = function() {
         alert(values[i]);
    }
}

由于JavaScript只有函数作用域，没有块作用域，所有事件处理程序共享相同的 i ，在循环结束后将具有值 10 。因此每个处理程序都会尝试提醒值[10] 。

As JavaScript has only function scope, no block scope, all the event handlers share the same i, which will have the value 10 after the loop finished. So every handler will try to alert values[10].

通过使用立即函数，新范围是介绍了捕获循环变量的当前值：

By using an immediate function, a new scope is introduced which "captures" the current value of the loop variable:

for(var i from 1..10) {
    (function(index) {
        elements[i].onclick = function() {
             alert(values[index]);
        }
    }(i));
}

由于这有时难以阅读，因此创建一个返回另一个函数的独立函数通常更好：

As this is sometimes hard to read, creating a standalone function which returns another function is often better:

function getHandler(value) {
     return function(){alert(value);};
}

for(var i from 1..10) {
     elements[i].onclick = getHandler(values[i]);
}

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