一句话概括，es6箭头函数里的this指的是定义这个函数时外层代码的this，这句话可以从两个方面理解：

1. es6箭头函数没有自己的this
2. es6箭头函数里的this是外层代码（定义时，非执行时）this的引用

我们用一个简单的例子理解一下

var Animal = function() {
    this.name = "Animal";
    this.speak = (words) => {
        console.log(this.name + ' is saying ' + words + '.');
    }
}
var cat = new Animal();
cat.speak("miao ~"); // Animal is saying miao ~.
var speak = cat.speak;
speak("miao ~"); // Animal is saying miao ~. In ES5, it should be undefined is saying miao~.

可以看到更改执行上下文，并没有影响到speak函数中的this指向。