Javascript this 关键字 详解

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


//<input type="text" value="3" id="txt_username">

$("#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());