ES6箭頭函式基本用法

``` window.onload = function(){
alert(abc);
}
//箭頭函式
window.onload = ()=>{
alert("abc");
} // 如果只有一個引數圓括號可以省
let play = function(num){
alert(num*2); //24
}
play(12);

let play = num => {
    alert(num*2);  //100
}
play(50);


//如果只有一個return  花括號可以省
let move = function(e){
    return e;
}
console.log(move(123)); //123

let move = num => num*2;

console.log(move(234)); //468
 

```