23.javascript 箭头函数

原先模式：

hello = function() {
  return "Hello World!";
}

箭头模式：

hello = () => {
  return "Hello World!";
}

//再变
hello = () => "Hello World!";

//有参数，再变
hello = (val) => "Hello " + val;

//最终变
hello = val => "Hello " + val;