enum

Enums allow us to define a set of named constants.

There are a few special rules about enum type:

values can be either string or number

enum map { a = 1, b = 'b' }

it is nominal typed

enum JapanVocaloid { miku, luka }

enum ChinaVocaloid { miku, LuoTianyi, XingChen }

let myFavoriteVocaloid = ChinaVocaloid.miku

// error
myFavoriteVocaloid = JapanVocaloid.miku

You should avoid using enum. You should use const instead.

// bad
enum Color { Red, Green, Blue }

// good
const COLOR = {
  Red: 0,
  Green: 1,
  Blue: 2
} as const

// good
const COLOR_RED = 0
const COLOR_GREEN = 1
const COLOR_BLUE = 2

Why?

While enum is more compact, and provide additional nominal protection, it is a TypeScript syntax. It goes against our design principles.

The transpiled code can be different depends on the config options, and it can get confusing as it may get erased (const enum).