es6 的 function, this 指向

es6 的 function
var fn = () => 'Hello';
即為
function fn () {
   return  'Hello';
}

如果只帶一個參數，可以拿掉parentheses()，如下
var fn = a => a+5;


this 指向
es5
function doSth(){
    console.log(this)
}

1.直接console.log function，結果是印出window物件
console.log(doSth())  //  [object Window] { ... }

2.直接在HTML的body內添加button tag，接著加上下面這行程式，結果是印出呼叫此function的物件→即object HTMLButtonElement
button.addEvenetListener('click', fn)//  [object HTMLButtonElement] { ... }

結論：this always refers to 呼叫此function的物件

es6

var fn2 = () => console.log(this);

button .addEventListener('click',fn2);  //  [object Window] { ... }

結論：this always refers to what it refers when you define the function no matter how or where you call the function