JavaScript“我”=“这个”，为什么？
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JavaScript "me" = "this", why?
提问by James Allardice
I saw in many source codes:
我在很多源代码中看到：
var me = this;
specially in Ext-JS 4 (JS framework). Why doing such thing? Is there any other reason or you just want for a variable to be called like "me" instead of "this"?
特别是在 Ext-JS 4（JS 框架）中。为什么要做这样的事情？是否有任何其他原因，或者您只想将变量称为“我”而不是“这个”？
Thank you.
谢谢你。
回答by James Allardice
Usually so you can keep a reference to thisinside a scope in which thisrefers to something else (like a callback function, for example).
通常，您可以this在一个范围内保留对其他内容的引用this（例如回调函数）。
Consider this example, in which the click event handler function has a different context to what you may expect (thisdoesn't refer to an instance of MyClass):
考虑这个例子，其中 click 事件处理函数具有与您预期不同的上下文（this不引用 的实例MyClass）：
var MyClass = function (elem) {
this.elem = elem;
this.name = "James";
elem.addEventListener("click", function () {
alert(this.name); //oops
}, false);
};
Now consider this example, in which we store a reference to the value of thisinside the constructor function, and use that inside the callback function:
现在考虑这个例子，我们this在构造函数内部存储了一个对 的值的引用，并在回调函数中使用它：
var MyClass = function (elem) {
var me = this;
this.elem = elem;
this.name = "James";
elem.addEventListener("click", function () {
alert(me.name); //works!
}, false);
};
The callback function can refer to a variable that was declared in the outer function, even after that function has returned (the MyClassconstructor returns as soon as it's executed the addEventListener). This is a demonstration of a closure.
回调函数可以引用在外部函数中声明的变量，即使在该函数返回之后（MyClass构造函数在执行后立即返回addEventListener）。这是一个闭包的演示。
回答by matt
Though of course closures are the more obvious reason for doing this, I just wanted to add that another reason can be to reduce the size of the minified version of a javascript file.
虽然当然闭包是这样做的更明显的原因，但我只想补充一点，另一个原因可能是减少 javascript 文件的缩小版本的大小。
thisas a keyword cannot be renamed in the process of minifying the file, while a local variable can. In other words, whenever you would use this (4 characters), instead a 1 character local variable can be used.
this作为关键字不能在缩小文件的过程中重命名，而局部变量可以。换句话说，无论何时使用它（4 个字符），都可以使用 1 个字符的局部变量。
Consider the following example functionof ExtJS's Ext.data.Store:
考虑以下ExtJS 的示例函数Ext.data.Store：
filterBy: function(fn, scope) {
var me = this;
me.snapshot = me.snapshot || me.data.clone();
me.data = me.queryBy(fn, scope || me);
me.fireEvent('datachanged', me);
me.fireEvent('refresh', me);
}
(note there's no closure involved here)
（注意这里不涉及闭包）
and its minified version:
及其缩小版本：
filterBy:function(b,a){var c=this;c.snapshot=c.snapshot||c.data.clone();c.data=c.queryBy(b,a||c);c.fireEvent("datachanged",c);c.fireEvent("refresh",c)}
(151 characters/bytes)
（151 个字符/字节）
Now, let's compare it to the minified version if we did not assign thisto a local variable:
现在，如果我们没有分配this给局部变量，让我们将它与缩小版本进行比较：
filterBy:function(b,a){this.snapshot=this.snapshot||this.data.clone();this.data=this.queryBy(b,a||this);this.fireEvent("datachanged",this);this.fireEvent("refresh",this)}
(170 characters/bytes)
（170 个字符/字节）
As you can see the version with a local variable only takes 88% of the size of the function which uses thiseach time instead.
正如您所看到的，带有局部变量的版本只占用this每次使用的函数大小的 88% 。
Especially in big libraries this can reduce the file size quite a bit.
特别是在大型库中，这可以大大减少文件大小。
回答by Nate
Setting me=thisallows you to use the thisvariable from an outer scope in an inner scope.
设置me=this允许您this在内部作用域中使用外部作用域中的变量。
var Outer= function () {
var me = this;
me.x = "outerx";
me.inner = {
x: "innerx",
displayValues: function () {
console.log(me.x); //outerx
console.log(this.x); //innerx
}
};
};
new Outer().inner.displayValues();
回答by Konstantin Dinev
Basically this utilizes closurein javascript. Read this about closure.
基本上这利用了 javascript 中的闭包。阅读有关关闭的内容。
It is used to carry the particular instance of thisto function calls where thishas a different meaning.
它用于承载具有不同含义的this函数调用的特定实例this。
