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枚举

枚举是组织收集有关联变量的一种方式。

数字枚举​

值为数字的枚举是双向映射的，可以从枚举成员映射到枚举值，也可以从枚举值映射到枚举成员。

enum Direction {
  NORTH,
  SOUTH,
  EAST,
  WEST
}

let northValue = Direction.NORTH;
console.log(northValue); // 0

let eastKey: string = Direction[2];
console.log(eastKey); // EAST

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 northValue = Direction.NORTH;
console.log(northValue); // 0
var eastKey = Direction[2];
console.log(eastKey); // EAST

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"

提示

js 赋值运算符返回的是被赋予的值。obj[k] = v 的返回值是 v，因此这里的 obj[obj[k] = v] = k 本质上就是进行了 obj[k] = v 与 obj[v] = k 这样两次赋值。

let obj = {};
let a = (obj['k'] = 1);
console.log(a); // 1
console.log((obj['foo'] = 1)); // 1

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。字符串枚举成员只会进行单次映射。

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

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

编译成 js 后：

var Direction;
(function (Direction) {
  Direction[(Direction['NORTH'] = 0)] = 'NORTH';
  Direction['SOUTH'] = 'south';
  Direction['EAST'] = 'east';
  Direction['WEST'] = 'west';
})(Direction || (Direction = {}));
console.log(Direction.SOUTH); // "south"

如果前面的枚举成员是字符串，那么后面的枚举成员需要赋值。下面的枚举成员 EAST 没有赋值，会报错。

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

常量枚举​

常量枚举的声明多了一个 const。

  • 只能通过枚举成员访问枚举值，而不能通过值访问成员
  • 在编译产物中不会存在一个额外的辅助对象（如下面的 Dir 对象）
  • 访问枚举成员会替换为枚举的值
const enum Dir {
  Foo,
  Bar,
  Baz
}
const r = Dir.Foo;

// 编译后的js
var r = 0; /* Dir.Foo */

如果使用普通枚举，编译后的产物复杂很多：

var Dir;
(function (Dir) {
  Dir[(Dir['Foo'] = 0)] = 'Foo';
  Dir[(Dir['Bar'] = 1)] = 'Bar';
  Dir[(Dir['Baz'] = 2)] = 'Baz';
})(Dir || (Dir = {}));
var r = Dir.Foo;

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，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 = {}));