this-4

ES6函数里的this指的是定义这个函数时外层代码的this，可以理解为:
1、ES6箭头函数没有自己的this；
2、ES6箭头函数的this是外层代码（定义时，非执行时，也就是词法作用域）this的引用。

<script>
    var Animal = function () {
        this.name = "Animal";
        this.speak = (words) => {
            console.log(this.name + " is saying " + words + ".");
        }
        this.diff = function (words) {
            console.log(this.name + " is saying " + words + ".");
        }
    };

    var cat = new Animal();
    cat.speak("miao!"); // Animal is saying miao!.
    cat.diff("common"); // Animal is saying common.
    console.log("------------------------");
    var speak = cat.speak;
    speak("miao!"); // Animal is saying miao!.
    var diff = cat.diff;
    diff("common"); //  is saying common.
</script>

《你不知道的JS》上卷this全面解析章节提到：

<script>
    function foo() {
        console.log(this.a); // 43
   }
    var a = 2; //声明在全局作用域中的变量（比如 var a = 2） 就是全局对象的一个同名属性
    window.a = 43;  // 没有这句的话会打印出2
    foo();
</script>