枚举（Enums）

数字枚举

enum Direction {
  Up = 1,
  Down,
  Left,
  Right,
}

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：

console.log(Direction.Up); // 1
console.log(Direction[1]); // Up

当然我们可以手动赋值：

enum Direction {
  Up = 10,
  Down,
  Left,
  Right,
}

此时，Up 为 10，Down 为 11，依次类推。

它会被编译为：

var Direction;
(function (Direction) {
  Direction[(Direction["Up"] = 10)] = "Up";
  Direction[(Direction["Down"] = 11)] = "Down";
  Direction[(Direction["Left"] = 12)] = "Left";
  Direction[(Direction["Right"] = 13)] = "Right";
})(Direction || (Direction = {})); // 这里的 Direction || (Direction = {}) 是为了防止重复声明

字符串枚举

enum Direction {
  Up = "UP",
  Down = "DOWN",
  Left = "LEFT",
  Right = "RIGHT",
}

假如从服务器获取到的数据是 UP，那么我们可以通过 Direction.Up 来获取到 UP。

const value = "UP";
if (value === Direction.Up) {
  // ...
}

常量枚举

const enum Direction {
  Up,
  Down,
  Left,
  Right,
}

使用常量枚举可以提升性能，因为它会在编译阶段被移除。

const enum Direction {
  Up,
  Down,
  Left,
  Right,
}

const value=1;
if (value === Direction.Up) {
  // ...
}

编译后：

const value = 1;
if (value === 0 /* Up */) {
  // ...
}