Switch cases exhaustive check in TypeScript like Elm does

One of the strong guarantees Elm offers is the exhaustive check that makes it impossible to forget to handle a possible branch in a case statement.

For instance, here:

type T = A | B

test =
    let
        c : T
        c = A
    in
    case c of
        A ->
            Debug.log "A" ""

        B -> 
            Debug.log "B" ""

If we don’t handle the B case the compiler complains:

This case does not have branches for all possibilities. Missing possibilities include: B

This is a nice feature, because if later on we add a new variant to the type T the compiler will remind us to handle the new branch in the case statement.

It is possible to have something very similar also with TypeScript, both with an eslint rule and with a clever use of never:

type T = 'a' | 'b'

const c: T = Math.random() > 0.5 ? 'a' : 'b'

switch (c) {
  case 'a':
    console.log('a')
    break
  case 'b':
    console.log('b')
    break
  default:
    const _exhaustiveCheck: never = c;
} 

If you forget one case (e.g. the second one) the compiler will report the error:

Type ‘“b”’ is not assignable to type ‘never’.

Check it out in this live example.