方法中的函數會掩蓋this，解決辦法！

要知道在javascript中this是種很神奇的東西，可是有時候也很淘氣；

以下：

<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>

此時this已經再也不指向obj了因此不會有name屬性；（this如今指向window，太淘氣了！）

對於這種狀況解決辦法以下：

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>