枚举

枚举是组织收集有关联变量的一种方式

数字枚举

enum Direction {
  NORTH,
  SOUTH,
  EAST,
  WEST
}
let dir: Direction = Direction.NORTH;
console.log(dir); // 0
let dirName: string = Direction[2];
console.log(dirName); // EAST
let taoWa = Direction[Direction[Direction.SOUTH]];
console.log(taoWa); // 1

被编译成 js 后的代码

var Direction;
(function (Direction) {
  Direction[(Direction['NORTH'] = 0)] = 'NORTH';
  Direction[(Direction['SOUTH'] = 1)] = 'SOUTH';
  Direction[(Direction['EAST'] = 2)] = 'EAST';
  Direction[(Direction['WEST'] = 3)] = 'WEST';
})(Direction || (Direction = {}));
var dir = Direction.NORTH;
console.log(dir); // 0
var dirName = Direction[2];
console.log(dirName); // EAST
var taoWa = Direction[Direction[Direction.SOUTH]];
console.log(taoWa); // 1

分析高亮行代码，Direction["NORTH"] = 0是将Direction对象中的NORTH属性值设为 0，接着执行Direction[0] = "NORTH"

tip

js 赋值运算符返回的是被赋予的值

let a = [];
function f() {
  return (a['b'] = 0);
}
f(); // 0

数字枚举默认第一个值是从 0 开始，后续依次递增 1，但是也可以改变任意枚举成员关联的数字

enum Direction {
  NORTH, // 0
  SOUTH = 3, // 3
  EAST, // 4
  WEST // 5
}

字符串枚举

枚举类型的值可以是字符串

enum Direction {
  NORTH = 'north',
  SOUTH = 'south',
  EAST = 'east',
  WEST = 'west'
}

console.log(Direction.NORTH); // "north"

常量枚举

enum Dir {
  False,
  True,
  Undefined,
  Null
}
const r = Dir.False;

编译成 js 后

var Dir;
(function (Dir) {
  Dir[(Dir['False'] = 0)] = 'False';
  Dir[(Dir['True'] = 1)] = 'True';
  Dir[(Dir['Undefined'] = 2)] = 'Undefined';
  Dir[(Dir['Null'] = 3)] = 'Null';
})(Dir || (Dir = {}));
var r = Dir.False;

可以看到const r = Dir.False编译后没啥区别，还是存在变量Dir。如果使用常量枚举可以看到简化了很多

const enum Dir {
  False,
  True,
  Undefined,
  Null
}
const r = Dir.False;

// 编译后的js
var r = 0; /* False */

枚举的静态方法

使用namespace可以给枚举类型添加静态方法

例子表示是否上班（不双休的就算了），注意export不可少

enum Weekday {
  Monday,
  Tuesday,
  Wednesday,
  Thursday,
  Friday,
  Saturday,
  Sunday
}

namespace Weekday {
  export function isBusinessDay(day: Weekday) {
    switch (day) {
      case Weekday.Saturday:
      case Weekday.Sunday:
        return false;
      default:
        return true;
    }
  }
}

const mon = Weekday.Monday;
const sun = Weekday.Sunday;

console.log(Weekday.isBusinessDay(mon)); // true
console.log(Weekday.isBusinessDay(sun)); // false

开放式枚举

可以拆分枚举块。在延续块中必须设置初始值，否则报错

enum Color {
  Red,
  Green,
  Blue
}

enum Color {
  DarkRed = 3,
  DarkGreen,
  DarkBlue
}

编译后

(function (Color) {
  Color[(Color['Red'] = 0)] = 'Red';
  Color[(Color['Green'] = 1)] = 'Green';
  Color[(Color['Blue'] = 2)] = 'Blue';
})(Color || (Color = {}));
(function (Color) {
  Color[(Color['DarkRed'] = 3)] = 'DarkRed';
  Color[(Color['DarkGreen'] = 4)] = 'DarkGreen';
  Color[(Color['DarkBlue'] = 5)] = 'DarkBlue';
})(Color || (Color = {}));