方法中的函数会掩盖this，解决办法！

要知道在javascript中this是种很神奇的东西，但是有时候也很淘气；

如下：

<script>
    var obj = {
        name: 'tqt',
        friends: ['shangs', 'lisi'],
        sayHello: function() {
            this.friends.forEach(function(friend){
                console.log(this.name + ' say hello to ' + friend);
            });
        },
    }
    obj.sayHello();
    //say hello to shangs
    //say hello to lisi
</script>

此时this已经不再指向obj了所以不会有name属性；（this现在指向window，太淘气了！）

对于这种情况解决办法如下：

方法一： 

<script>
    var obj = {
        name: 'tqt',
        friends: ['shangs', 'lisi'],
        sayHello: function() {
            var that = this;
            this.friends.forEach(function(friend){
                console.log(that.name + ' say hello to ' + friend);
            });
        },
    }
    obj.sayHello();
    //tqt say hello to shangs
    //tqt say hello to lisi
</script>

方法二：

<script>
    var obj = {
        name: 'tqt',
        friends: ['shangs', 'lisi'],
        sayHello: function() {
            this.friends.forEach(function(friend){
                console.log(this.name + ' say hello to ' + friend);
            }.bind(this));
        },
    }
    obj.sayHello();
    //tqt say hello to shangs
    //tqt say hello to lisi
</script>

方法三：

<script>
    var obj = {
        name: 'tqt',
        friends: ['shangs', 'lisi'],
        sayHello: function() {
            this.friends.forEach(function(friend){
                console.log(this.name + ' say hello to ' + friend);
            }, this);
        },
    }
    obj.sayHello();
    //tqt say hello to shangs
    //tqt say hello to lisi
</script>