Class UnreachableCaseError

An thin wrapper around KindError which can be used to ensure exhaustiveness of switch-case statements

Param

The variable being refined by the switch-case statement.

Example:

import {UnreachableCaseError} from "@khanacademy/wonder-stuff-core";
type Action =
  | {type: "insert"; id: string; value: any}
  | {type: "delete"; id: string};

function handleAction(action: Action) {
  switch (action.type) {
    case "insert": {
      // do insert
      break;
    }
    case "delete": {
      // do delete
      break;
    }
    default: {
      throw new UnreachableCaseError(action);
    }
  }
}
Copy

If a new action type is added to the Action type that isn't handled by the switch-case statement, the default case will become reachable and TypeScript will report an error. This is becasue the first parameter of UnreachableCaseError's constructor is typed as never and nothing can be assigned (or passed) to something typed as never.