Peter Mekhaeil

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.