为什么代码 this 指向 window 对象? [英] why the code this point to window object?
问题描述
我的代码是:
var length = 20;
function fn(){
console.log(this.length);
}
var o = {
length:10,
e:function (fn){
fn();
arguments[0]();
}
}
o.e(fn);
输出是20,1
,谁能告诉我为什么?
the output is 20,1
,who can tell me why?
推荐答案
当 this
关键字出现在函数内部时,其值取决于函数的调用方式.
When the this
keyword occurs inside a function, its value depends on how the function is called.
在您的情况下,fn()
被调用时没有提供 this 值,因此默认值为 window
.对于arguments[0]()
,上下文是arguments
对象,其长度为1
.
In your case, fn()
is called without providing the a this value, so the default value is window
.
With arguments[0]()
, the context is the arguments
object, whose length is 1
.
关键在于函数在哪里调用并不重要,重要的是函数如何调用.
The point is it does not matter where the function is called, but it matters how the function is called.
var length = 20;
function fn(){
console.log(this.length);
}
var o = {
length:10,
e:function (fn){
fn(); // this will be the window.
arguments[0](); // this will be arguments object.
}
}
o.e(fn);
此外,如果你想让this
成为对象o
,你可以使用call
或apply
, 或者 bind
先绑定一个对象.
Further more, if you want this
to be the object o
, you could use call
or apply
, or bind
an object first.
var length = 20;
function fn(){
console.log(this.length);
}
var o = {
length:10,
e:function (fn){
var fn2 = fn.bind(this);
fn.call(this); // this in fn will be the object o.
fn.apply(this); // this in fn will be the object o.
fn2(); // this also will be the object o.
}
}
o.e(fn);
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