Use an Enum as Object Key in TypeScript

Objects are a fast and convenient data type in JavaScript. You often want to put a restriction on what keys can be used. You can use TypeScript enums in two ways to define the keys in an object.

Using the in nomenclature:

enum Animal {
  Bird,
  Cat,
  Dog,
}

type AnimalCount = { [key in Animal]: number }

const animals: AnimalCount = {
  [Animal.Bird]: 1,
  [Animal.Cat]: 3,
  [Animal.Dog]: 5,
}

The other syntax creates a slightly different object:

enum Animal {
  Bird,
  Cat,
  Dog,
}

type AnimalCount = { [key in keyof typeof Animal]: number }

const animals: AnimalCount = {
  Bird: 1,
  Cat: 3,
  Dog: 5,
}

The keys are limited by the enum keys, but you can't use [Animal.Bird] as a key.