es比较有用的新特性

1、防止崩溃的可选链（?）

在未定义的属性时使用可选链运算符，将返回undefined而不会报错。可防止代码崩溃

const student = {
  name: "Matt",
  age: 27,
  address: {
    state: "New York"
  },
};


// LONG FORM
console.log(student && student.address && student.address.ZIPCode); // Doesn't exist - Returns undefined


// SHORTHAND
console.log(student?.address?.ZIPCode); // Doesn't exist - Returns undefined

2、指数运算符 （**）

Math.pow(2,4) === 2**4

3.四舍五入的Math.floor()简写

Math.floor(5.23) === ~~5.23 === 5