[TypeScript] Creating a literal type union from object values

Let's say you have the following JavaScript object:

const test = {
  a: "hello",
  b: "world",
  c: 0
}

And you want to create a literal type union using the values of this object, namely "hello", "world", and 0.

type TestValues = "hello" | "world" | 0

The answer would be as follows:

const test = {
  a: "hello",
  b: "world",
  c: 0
} as const // <== !!!

type TestValues = (typeof test)[keyof typeof test]
// type TestValues = 0 | "hello" | "world"

Make sure to include as const.

If you forget to add it, the resulting type union would be a union of string and number, rather than the exact literal values.

const test = {
  a: "hello",
  b: "world",
  c: 0
}

type TestValues = (typeof test)[keyof typeof test]
// type TestValues = string | number

By the way, if you want to create a similar type based on the keys of the object instead of the values, you can do it like this:

const test = {
  a: "hello",
  b: "world",
  c: 0
}

type TestKeys = keyof typeof test
// type TestKeys = "a" | "b" | "c"