严格模式下

1.全局作用域指向当前Window

2.函数内部的this，运行在全局作用域下，this值为undefined

3.立即执行函数的 this值为undefined

          "use strict"var sb='sb';//全局作用下的thisconsole.log(this);//Windowfunction sayHello(){console.log('function');//console.log(this);//undefined}//运行在全局作用域下的函数sayHello();//匿名立即执行函数(function(){console.log('anonymous');console.log(this);//undefined})();

在非严格模式下

在该模式下**this** 引用总是指向一个值，该值总是指向持有**this**的 **Funtion或者Object **所运行的作用域

   var firstName="Peter",lastName='Ally';function showFullName(){//持有this的Funtion showFullName运行在全局作用域下，所以this指向Window//输出PeterAllyconsole.log(this.firstName+""+this.lastName);}    showFullName(); var person={firstName:"Penelope",lastName:'Barrymore',showFullName:function(){//持有this的Funtion showFullName运行在person作用域下，所以this指向person//输出PenelopeBarrymoreconsole.log(this.firstName+""+this.lastName);}}person.showFullName();

一些特殊情况

1.我们可以利用**Bind ()**, Apply (), **Call ()重新指定this**引用

         var person={firstName:"Penelope",lastName:'Barrymore',showFullName:function(){//持有this的Funtion showFullName运行在person作用域下，所以this指向person//输出PenelopeBarrymoreconsole.log(this.firstName+""+this.lastName);}}//person.showFullName();var anotherPerson={firstName:'Rohit',lastName:'Khan'}//重新指定this为anotherPerson//输出为RohitKhanperson.showFullName.apply(anotherPerson);

2.回调函数中的**this**

              var user={data:[{name:"T.Woods",age:37},{name:"P. Mickelson",age:43}],clickHandler:function(event){var randomNum = ((Math.random () * 2 | 0) + 1) - 1; // random number between 0 and 1​console.log (this.data[randomNum].name + " " + this.data[randomNum].age);}}$ ("button").click (user.clickHandler);

**button的click事件击发的时候this并不能访问到 user的属性data,因为此时的this**引用的是 **$ ("button")**对象

修正如下：

 $("button").click (user.clickHandler.bind (user));

3.闭包

               var user={tournament:"The Masters",data:[{name:"T.Woods",age:37},{name:"P. Mickelson",age:43}],clickHandler:function(event){this.data.forEach(function(person){//这是一个内部函数this不再指向user//this只能被Function,Object自身访问console.log ("What is This referring to? " + this); //[objectWindow]console.log (person.name + " is playing at " + this.tournament);// T. Woods is playing at undefined​// P. Mickelson is playing at undefined​​})}}user.clickHandler();

修正如下:

                  clickHandler:function(event){var theUserObj = this;this.data.forEach(function(person){//这是一个内部函数this不再指向user//this只能被Function,Object自身访问console.log ("What is This referring to? " + this); //[objectWindow]console.log (person.name + " is playing at " + theUserObj.tournament);// T. Woods is playing at undefined​// P. Mickelson is playing at undefined​​})}

在外部函数中定义一个变量保存**this，在闭包中通过访问这个变量访问外部的this**

4.将方法赋值给变量

var data=[{name:"Samantha",age:12},{name:"Alexis",age:14}];var user={tournament:"The Masters",data:[{name:"T.Woods",age:37},{name:"P. Mickelson",age:43}],showData:function(event){var randomNum = ((Math.random () * 2 | 0) + 1) - 1;// random number between 0 and 1console.log (this.data[randomNum].name + " " + this.data[randomNum].age);}}var showUserData = user.showData;showUserData (); // Samantha 12 (from the global data array)​

最终输出的数据为全局变量中**data**的数据

修正如下:

var showUserData = user.showData.bind (user);