javascript 为什么我不能在原型函数中为“this”分配一个新值？
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Why can't I assign a new value to "this" in a prototype function?
提问by Philip Walton
Why can I do this:
为什么我可以这样做：
Array.prototype.foo = function() {
this.splice(0, this.length);
return this.concat([1,2,3]);
}
But I can't do this:
但我不能这样做：
Array.prototype.foo = function() {
return this = [1,2,3];
}
Both functions destroy the value of this and change it to [1,2,3]but the second one throws the following error: Uncaught ReferenceError: Invalid left-hand side in assignment
这两个函数都会破坏 this 的值并将其更改为[1,2,3]但第二个会引发以下错误：Uncaught ReferenceError: Invalid left-hand side in assignment
I suspect it's because allowing assignment means I could potentially change the array to something else (like a string), but I'm hoping someone out there knows for sure and/or has a more detailed explanation.
我怀疑这是因为允许分配意味着我可以将数组更改为其他内容（如字符串），但我希望那里有人肯定知道和/或有更详细的解释。
采纳答案by Greg Hewgill
It's not permitted to assign a value to thiswithin a function. Suppose that you coulddo this, and your code looked something like:
不允许this在函数内赋值。假设您可以这样做，并且您的代码如下所示：
Array.prototype.foo = function() {
return this = [1, 2, 3];
}
var a = ["beans", "rice"];
a.foo();
// a now points to an object containing [1, 2, 3]
Now, what if you did this:
现在，如果你这样做：
var a = ["beans", "rice"];
var b = a; // b refers to the same object as a
b.foo();
// what does b refer to now? how about a?
The act of calling a function .foo()on an object should not change the identityof the object. This would be very confusing for the caller if bsuddenly started referring to a different object than asimply because some method was called.
.foo()在对象上调用函数的行为不应改变对象的身份。如果b突然开始引用不同的对象，而a不仅仅是因为调用了某个方法，这对于调用者来说会非常混乱。
回答by Tobias Cohen
You're confusing objects with references.
您将对象与引用混淆了。
An array is an object, when you use a literal like [1,2,3]you're making a new array.
数组是一个对象，当您像[1,2,3]创建新数组一样使用文字时。
A variable name like thisor ais a reference to an object. If it helps, imagine an object as a person, and the reference as their nickname. You can have more than one reference to the same object, for example:
变量名如this或a是对对象的引用。如果有帮助，把一个对象想象成一个人，把引用想象成他们的昵称。你可以有多个对同一个对象的引用，例如：
var a = [1,2];
var b = a;
b.push(3);
alert(a.length); // Gives "3"
Imagine if you had a friend named Sarah. You also sometimes call her "Ace". If Sarah gets a haircut one day, Ace has a haircut too, because "Sarah" and "Ace" are both different names for the same person.
想象一下，如果你有一个叫莎拉的朋友。你有时也称她为“Ace”。如果有一天莎拉理了发，艾斯也会理发，因为“莎拉”和“艾斯”对于同一个人来说都是不同的名字。
If you use a mutating array method like a.pushor a.splice(concathowever is not one!), you change the existing Array object, and astill refers to the same object. This is like getting a hair cut - you may look different, but you're still the same person.
如果您使用像a.push或这样的变异数组方法a.splice（concat但不是一个！），您会更改现有的 Array 对象，并且a仍然引用同一个对象。这就像理发——你可能看起来不同，但你仍然是同一个人。
When you assign a reference to a new value, with a = [1,2,3], you're creating a new array, and changing ato refer to it. This is like finding a new friend with different hair, and deciding to call her Ace instead.
当您将引用分配给新值时，使用a = [1,2,3]，您正在创建一个新数组，并更改a为引用它。这就像找到一个头发不同的新朋友，并决定称她为 Ace。
Now thisis a special name generated by Javascript. It's not a given name like Sarah, but more of a title, like "mother". Your mother can get a new haircut, but you can't get a new mother. Likewise, you can't change what thisrefers to from inside a function.
现在this是由 Javascript 生成的特殊名称。它不是像莎拉这样的给定名字，而更像是一个头衔，比如“母亲”。你妈妈可以换个新发型，但你不能找一个新妈妈。同样，您不能this从函数内部更改引用的内容。
回答by xdazz
You are not allowed to re-assign this. Because the thisvalue associated with an execution context is immutable.
您不得重新分配this。因为与执行上下文关联的this值是不可变的。
回答by Avenida Gez
Is it possible to change the value of this?Not by assignment, you cannot assign a value to this
是否可以更改 的值this？不是通过赋值，你不能赋值给this
Your case:
你的情况：
Array.prototype.foo = function() { this.splice(0, this.length); return this.concat([1,2,3]); }But I can't do this:
Array.prototype.foo = function() { return this = [1,2,3]; }
Array.prototype.foo = function() { this.splice(0, this.length); return this.concat([1,2,3]); }但我不能这样做：
Array.prototype.foo = function() { return this = [1,2,3]; }
When you modify the prototype adding method foo(), the first part makes the array become empty, equals to reassign its instance.
修改原型添加方法时foo()，第一部分使数组变空，等于重新分配其实例。
For example to var a = []
例如到 var a = []
Then in the return value:
然后在返回值中：
return this.concat([1,2,3]);
Remember that concatdoes not destroys an array, it creates a new array and returns it
请记住，concat这不会破坏数组，它会创建一个新数组并返回它
Given a = ['x']and b = ['y'], c = a.concat(b)results in c = ['x','y'], and aand bremain the same as they were
给定a = ['x']and b = ['y'],c = a.concat(b)结果c = ['x','y'], aandb保持不变
Now since you are doing this inside the array
现在既然你在数组里面做这个
a=['a','b','c']
then you call a.foo();, inside awill get equal a=[], with this.splice(0, this.length);then will concatenate [ ]with [1,2,3]
然后你打电话a.foo();，里面a会得到相等a=[]，this.splice(0, this.length);然后将[ ]与[1,2,3]
So if I say just a.foo(), in fact nothing happens outside if I do not assign the result to something, just a remains empty.
因此，如果我说 just a.foo()，实际上，如果我不将结果分配给某事，则外部不会发生任何事情，只是 a 保持为空。
If ais assigned to something
如果a被分配给某物
c = a.foo()
then cwill be [1,2,3], and astill empty [ ]
然后c将是[1,2,3]，a仍然是空的[ ]
In the second part, this = [1,2,3]
在第二部分， this = [1,2,3]
It is possible to change the value of this?, not by assignment, you cannot assign a value to this
是否可以更改 this 的值？，不是通过赋值，您不能为 this 赋值
