DIV CSS 佈局教程網

看如下對象定義：

'use strict'
var jane = {
  name ： ‘Jane',
  display : function(){
    retrun 'Person named ' + this.name;
  }
};

這樣能正常調用

jane.display();

下面的調用會出錯：

var func = jane.display;
func()

TypeError: Cannot read property 'name' of undefined

因為，this指向已經改變，正確的方式如下：

var func2 = jane.display.bind(jane);
func2()

'Penson named Jane'

所有函數都有其特殊的this變量，如下面的forEach

var jane = {
  name : 'Jane',
  friends: ['Tarzan', 'Cheeta'],
  sayHiToFriends: function(){
    'use strict';
    this.friends.forEach(function(friend) {
      // 'this' is undefined here
      console.log(this.name + ' says hi to '+ friend);
    });
  }
}  

調用sayHiToFriends會產生一個錯誤：

jane.sayHiToFriends()

TypeError: Cannot read property 'name' of undefined

解決方案一：將this保存在不同的變量中

var jane = {
  name : 'Jane',
  friends: ['Tarzan', 'Cheeta'],
  sayHiToFriends: function(){
    'use strict';
    var that = this;
    this.friends.forEach(function(friend) {
      console.log(that.name + ' says hi to '+ friend);
    });
  }
} 

解決方案二：利用forEach的第二個參數，它可以給this指定一個值

var jane = {
  name : 'Jane',
  friends: ['Tarzan', 'Cheeta'],
  sayHiToFriends: function(){
    'use strict';
    this.friends.forEach(function(friend) {
      console.log(this.name + ' says hi to '+ friend);
    }, this);
  }
}