什么'var that = this;'用JavaScript表示？ [英] What does 'var that = this;' mean in JavaScript?

问题描述

在我看到的JavaScript文件中：

In a JavaScript file I saw:

function Somefunction(){
   var that = this; 
   ... 
}

声明<$ c的目的是什么$ c>那并将其分配给这个？

推荐答案

我将以插图的方式开始这个答案：

I'm going to begin this answer with an illustration:

var colours = ['red', 'green', 'blue'];
document.getElementById('element').addEventListener('click', function() {
    // this is a reference to the element clicked on

    var that = this;

    colours.forEach(function() {
        // this is undefined
        // that is a reference to the element clicked on
    });
});

我的回答最初用jQuery证明了这一点，只是略有不同：

My answer originally demonstrated this with jQuery, which is only very slightly different:

$('#element').click(function(){
    // this is a reference to the element clicked on

    var that = this;

    $('.elements').each(function(){
        // this is a reference to the current element in the loop
        // that is still a reference to the element clicked on
    });
});

因为这个经常在你改变时改变通过调用新函数的范围，您无法使用它来访问原始值。将其别名为，允许您仍然可以访问的原始值。

Because this frequently changes when you change the scope by calling a new function, you can't access the original value by using it. Aliasing it to that allows you still to access the original value of this.

就个人而言，我不喜欢使用那个作为别名。很少有人明白它的含义，特别是如果函数长于几行。我总是使用更具描述性的别名。在上面的例子中，我可能会使用 clickedEl 。

Personally, I dislike the use of that as the alias. It is rarely obvious what it is referring to, especially if the functions are longer than a couple of lines. I always use a more descriptive alias. In my examples above, I'd probably use clickedEl.

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