javascript箭头函数

let func = (arg1, arg2, ..., argN) => expression

let func = function(arg1, arg2, ..., argN) {
  return expression;
};

let sum = (a, b) => a + b;
/* This arrow function is a shorter form of:
let sum = function(a, b) {
  return a + b;
};
*/
alert( sum(1, 2) ); // 3

let double = n => n * 2;
// roughly the same as: let double = function(n) { return n * 2 }
alert( double(3) ); // 6

let sayHi = () => alert("Hello!");
sayHi();