枚举（Enum）类型用于取值被限定在一定范围内的场景，比如一周只能有七天，颜色限定为红绿蓝等。

  1. enum Days {Sun, Mon, Tue, Wed, Thu, Fri, Sat};
  3. // 会编译为
  4. var Days;
  5. (function (Days) {
  6.     Days[Days["Sun"] = 0] = "Sun";
  7.     Days[Days["Mon"] = 1] = "Mon";
  8.     Days[Days["Tue"] = 2] = "Tue";
  9.     Days[Days["Wed"] = 3] = "Wed";
  10.     Days[Days["Thu"] = 4] = "Thu";
  11.     Days[Days["Fri"] = 5] = "Fri";
  12.     Days[Days["Sat"] = 6] = "Sat";
  13. })(Days || (Days = {}));

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. }
  7. let directions = [
  8.   Directions.Up, Directions.Down, Directions.Left, Directions.Right
  9. ];
  11. // 会编译为
  12. var directions = [0 /* Up */, 1 /* Down */, 2 /* Left */, 3 /* Right */];

外部枚举

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

  1. declare enum Directions {
  2.     Up,
  3.     Down,
  4.     Left,
  5.     Right
  6. }
  7. let directions = [
  8.   Directions.Up, Directions.Down, Directions.Left, Directions.Right
  9. ];
  11. // 
  12. // 会编译为
  13. var directions = [
  14.   Directions.Up, Directions.Down, Directions.Left, Directions.Right
  15. ];