看如下对象定义：
'use strict'
var jane = {
name ： ‘Jane',
display : function(){
retrun 'Person named ' + this.name;
}
};
这样能正常调用
jane.display();
下面的调用会出错：
var func = jane.display; func()
TypeError: Cannot read property 'name' of undefined
因为，this指向已经改变，正确的方式如下：
var func2 = jane.display.bind(jane); func2()
'Penson named Jane'
所有函数都有其特殊的this变量，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);
});
}
}
调用sayHiToFriends会产生一个错误：
jane.sayHiToFriends()
TypeError: Cannot read property 'name' of undefined
解决方案一：将this保存在不同的变量中
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);
});
}
}
解决方案二：利用forEach的第二个参数，它可以给this指定一个值
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);
}
}