Enums 枚举

Numeric enums

enum Direction {
    // 不写 1 的话默认从 0 递增
    Up = 1,
    Down,
    Left,
    Right,
}

String enums

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

Heterogeneous enums (mixed)

可以用但最好不要用

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

Computed and constant members

每个 enum member 都有值, 要么是一个 constant, 要么是一个 computed

enum FileAccess {
    // constant members
    None,
    Read    = 1 << 1,
    Write   = 1 << 2,
    ReadWrite  = Read | Write,
    // computed member
    G = "123".length
}

Union enums and enum member types

枚举和枚举成员都可以当作类型使用

Enums at runtime, Reverse mappings

枚举最终会编译成 js 的一个对象,
对于 numeric enum 来说, 会进行 reverse mapping, 就是 k-v 或者 v-k 都可以访问到
string enum 不会进行 reverse mapping

// ts
enum E {
    X, Y, Z
}
// js
var E;
(function (E) {
    E[E["X"] = 0] = "X";
    E[E["Y"] = 1] = "Y";
    E[E["Z"] = 2] = "Z";
})(E || (E = {}));

const enums

// ts
const enum Directions {
    Up,
    Down,
    Left,
    Right
}
let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right]
// js
// const enum 在编译后不会生成一个对象, 使用的地方会之直接被 inlined 写死进去\
// 所以 const enum 只能使用 constant, 不能使用 computed
var directions = [0 /* Up */, 1 /* Down */, 2 /* Left */, 3 /* Right */];

Ambient enums

Ambient enums are used to describe the shape of already existing enum types.

不晓得什么鬼意思