枚举

数字枚举：

enum Direction {
    Up = 1, // 也可以不写，直接默认0开始
    Down,
    Left,
    Right
}

字符串枚举：

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

异构枚举：数字和字符串混合

enum BooleanLikeHeterogeneousEnum {
    No = 0,
    Yes = "YES",
}