枚举项有两种类型：常数项（constant member）和计算所得项（computed member）。
又分为 普通枚举 常数枚举 外部枚举

常数项

枚举（Enum）类型用于取值被限定在一定范围内的场景

  1. enum Days {Sun, Mon, Tue, Wed, Thu, Fri, Sat};

枚举成员会被赋值为从 0 开始递增的数字，同时也会对枚举值到枚举名进行反向映射:

  1. var Days;
  2. (function (Days) {
  3.     Days[Days["Sun"] = 0] = "Sun";
  4.     Days[Days["Mon"] = 1] = "Mon";
  5.     Days[Days["Tue"] = 2] = "Tue";
  6.     Days[Days["Wed"] = 3] = "Wed";
  7.     Days[Days["Thu"] = 4] = "Thu";
  8.     Days[Days["Fri"] = 5] = "Fri";
  9.     Days[Days["Sat"] = 6] = "Sat";
  10. })(Days || (Days = {}));

手动赋值

  1. enum Days {
  2.   Sun = 3,
  3.   Mon = 1,
  4.   Tue,
  5.   Wed,
  6.   Thu,
  7.   Fri,
  8.   Sat
  9. }
  11. console.log(Days[3] == 'Sun'); // false
  12. console.log(Days['Sun']); //3

因为手动赋值的缘故 导致Days[3]的值先是”Sun” 而后又被”Wed”覆盖了

  1. var Days;
  2. (function (Days) {
  3.     Days[Days["Sun"] = 3] = "Sun";
  4.     Days[Days["Mon"] = 1] = "Mon";
  5.     Days[Days["Tue"] = 2] = "Tue";
  6.     Days[Days["Wed"] = 3] = "Wed";
  7.     Days[Days["Thu"] = 4] = "Thu";
  8.     Days[Days["Fri"] = 5] = "Fri";
  9.     Days[Days["Sat"] = 6] = "Sat";
  10. })(Days || (Days = {}));

手动赋值的枚举项也可以为小数或负数，此时后续未手动赋值的项的递增步长仍为 1 (小数会出现精度丢失的问题 慎用)

手动赋值的枚举项可以不是数字，此时需要使用类型断言来让 tsc 无视类型检查 (编译出的 js 仍然是可用的)：

  1. enum Days {Sun = 7, Mon, Tue, Wed, Thu, Fri, Sat = <any>"S"};
  4. ---------
  5.   var Days;
  6. (function (Days) {
  7.     Days[Days["Sun"] = 7] = "Sun";
  8.     Days[Days["Mon"] = 8] = "Mon";
  9.     Days[Days["Tue"] = 9] = "Tue";
  10.     Days[Days["Wed"] = 10] = "Wed";
  11.     Days[Days["Thu"] = 11] = "Thu";
  12.     Days[Days["Fri"] = 12] = "Fri";
  13.     Days[Days["Sat"] = "S"] = "Sat";
  14. })(Days || (Days = {}));

如果不是数字 则该项后面的每一个枚举项都要初始化

计算所得项

  1. enum Color {Red, Green, Blue = "blue".length};

“blue”.length 就是一个计算所得项
如果紧接在计算所得项后面的是未手动赋值的项，那么它就会因为无法获得初始值而报错：

当满足以下条件时，枚举成员被当作是常数:

常数枚举

常数枚举是使用 const enum 定义的枚举类型：

  1. const enum Directions {
  2.     Up,
  3.     Down,
  4.     Left,
  5.     Right
  6. }
  8. let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];

常数枚举与普通枚举的区别是，它会在编译阶段被删除，并且不能包含计算成员。
编译结果：

  1. var directions = [0 /* Up */, 1 /* Down */, 2 /* Left */, 3 /* Right */];

假如包含了计算成员，则会在编译阶段报错：

  1. const enum Color {Red, Green, Blue = "blue".length};
  3. // index.ts(1,38): error TS2474: In 'const' enum declarations member initializer must be constant expression.

外部枚举

外部枚举（Ambient Enums）是使用 declare enum 定义的枚举类型：

declare 定义的类型只会用于编译时的检查，编译结果中会被删除。

  1. declare enum Directions {
  2.     Up,
  3.     Down,
  4.     Left,
  5.     Right
  6. }
  8. let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];

编译结果:

  1. var directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];

同时使用 declare 和 const 也是可以的

  1. declare const enum Directions {
  2.     Up,
  3.     Down,
  4.     Left,
  5.     Right
  6. }
  8. let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];

编译结果：

  1. var directions = [0 /* Up */, 1 /* Down */, 2 /* Left */, 3 /* Right */];