TypeScript 3.4 で Array (string[]) を Union Type に変換する Tips

参考: https://stackoverflow.com/questions/45251664/typescript-derive-union-type-from-tuple-array-values

TypeScript 3.4 で組み込まれた as const を使ってより簡潔に実装できるようになった

const directionList = ['east', 'west', 'south', 'north'] as const
// -> Readonly ['east', 'west', 'south', 'north']

type Direction = typeof directionList[number]
// -> 'east' | 'west' | 'south' | 'north'

独自定義の Type Guard と組み合わせたりするのに便利そう

const isDirection = (obj: any): obj is Direction => {
  return directionList.includes(obj)
}

const doSomething = (data: any) => {
  // validation
  if (!isDirection(data.direction)) {
    throw new Error('direction is not found')
  }

  // do something
}