Javascript this 关键字 详解_基础知识

一、this指向构造函数实例化对象

在上篇文章中，我们提到了使用new和不使用new调用构造函数的区别，如下例：

function Benjamin(username, sex) {
this.username = username;
this.sex = sex;
}
var benjamin = new Benjamin("zuojj", "male");
//Outputs: Benjamin{sex: "male",username: "zuojj"}
console.log(benjamin);
var ben = Benjamin("zhangsan", "female");
//Outputs: undefined
console.log(ben);

当构造函数当做普通函数被调用时，并没有返回值，同时this指向全局对象。那么我们如何来避免因为缺少new关键字，而产生的问题呢？

function Benjamin(username, sex) {
//Check whether "this" is a "Benjamin" object
if(this instanceof Benjamin) {
this.username = username;
this.sex = sex;
}else {
return new Benjamin(username, sex);
}
}
var benjamin = new Benjamin("zuojj", "male");
//Outputs: Benjamin{sex: "male",username: "zuojj"}
console.log(benjamin);
var ben = Benjamin("zhangsan", "female");
//Outputs: Benjamin {username: "zhangsan", sex: "female"}
console.log(ben);

在上例中，我们首先检查this是否是Benjammin的实例，如果不是，使用new自动调用构造函数，并实例化，这意味着，我们不再需要担心，遗漏new关键字实例化构造函数。当然这样我们可能会养成一个坏的习惯，如果避免这种现象呢？我们可以抛出一个错误，像下面这样：

function Benjamin(username, sex) {
//Check whether "this" is a "Benjamin" object
if(this instanceof Benjamin) {
this.username = username;
this.sex = sex;
}else {
// If not, throw error.
throw new Error("`Benjamin` invoked without `new`");
}
}

二、this指向调用该函数的对象

看下面的例子:

var x = 10;
var obj = {
x: 10,
output: function() {
//Outputs: true
console.log(this === obj);
return this.x;
},
innerobj: {
x: 30,
output: function() {
//Outputs: true
console.log(this === obj.innerobj);
return this.x;
}
}
};
//Outputs: 10
console.log(obj.output());
//Outputs: 30
console.log(obj.innerobj.output());

三、this指向全局对象

在上面讨论构造函数的时候我们也讨论到不适用new的时候，this会指向全局对象，下面我们来看看两种常见的容易犯错的实例：

var x = 100;
var obj = {
x: 10,
output: function() {
(function() {
//Outputs: true
console.log(this === window);
//Outputs: Inner: 100
console.log("Inner:" + this.x);
})();

return this.x;
}
};
//Outputs: 10
console.log(obj.output());

在使用闭包的时候，作用域发生变化，this指向window(浏览器中)。

var x = 100;
var obj = {
x: 10,
output: function() {
return this.x;
}
};
var output = obj.output;
//Outputs: 10
console.log(obj.output());
//Outputs: 100
console.log(output());
var obj2 = {
x: 30,
output: obj.output
}
//Outputs: 30
console.log(obj2.output());

此时this始终指向函数调用时的对象。

四、this指向apply/call()方法指派的对象

var x = 100;
var obj = {
x: 10,
output: function() {
return this.x;
}
};
//Outputs: 10
console.log(obj.output());
var obj2 = {
x: 40,
output: obj.output
}
//Outputs: 40
console.log(obj.output.call(obj2));
//Outputs: 10
console.log(obj2.output.apply(obj));

五、callback函数內的this指向调用该callback的函数的this所指向的对象

//
$("#username").on("click", function() {
console.log(this.value);
});

六、Function.prototype.bind中的this

The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
实例一：

function person() {
return this.name;
}
//Function.prototype.bind
var per = person.bind({
name: "zuojj"
});
console.log(per);
var obj = {
name: "Ben",
person: person,
per: per
};
//Outputs: Ben, zuojj
console.log(obj.person(), obj.per());

实例二：

this.x = 9;
var module = {
x: 81,
getX: function() { return this.x; }
};
//Outputs: 81
console.log(module.getX());
var getX = module.getX;
//Outputs: 9, because in this case, "this" refers to the global object
console.log(getX);
// create a new function with 'this' bound to module
var boundGetX = getX.bind(module);
//Outputs: 81
console.log(boundGetX());