TypeScript: Exhaustiveness checking in switch with union type

The never data type in TypeScript can be used to check that all cases in a switch are considered.

type Day =
| "Monday"
| "Tuesday"
| "Wednesday"
| "Thursday"
| "Friday"
| "Saturday"
| "Sunday";

function getDayIndex(day: Day) {
switch (day) {
case "Monday": {
return 1;
default: {
// `Type 'string' is not assignable to type 'never'.`
const _exhaustiveCheck: never = day;
return _exhaustiveCheck;

_exhaustiveCheck will have an error because TypeScript is attempting to assign the rest of the Day union to never which cannot happen.

This can be found in the TypeScript documentation.