枚举类型的枚举值默认是1, 2, 3

enum Direction {
  up,
  down,
  left,
  right,
}
console.log(Direction.down);    // 1
console.log(Direction[0]);    // 这个值反解回来就是"up"这个字符串

自定义枚举值

enum Direction {
  up = "a",
  down = "b",
  left = "c",
  right = "d",
}
console.log(Direction.down);    // a
console.log(Direction["a"]);    // up
console.log(Direction["a"] === "up");     //    true

常量枚举

常量枚举可以提升性能, 但是不能反解

const enum Direction {
  up,
  down,
  left,
  right,
}
console.log(Direction.down);
console.log(Direction[1]);