[TypeScript] Use TypeScript’s never Type for Exhaustiveness Checking

Posted 2020-10-13 Answer1215

TypeScript 2.0 introduced a new primitive type called never, the type of values that never occur. It helps model the completion behavior of functions more accurately and can also be used for exhaustiveness checking.

 

In somecases, never type can be useful as well.

enum ShirtSize {
  XS,
  S,
  M,
  L,
  XL
}

function assertNever(value: never): never {
  // throw Error(`Unexpected value ‘${value}‘`)
  // Adjusted for plunker output
  console.log(Error(`Unexpected value ‘${value}‘`));
}

function prettyPrint(size: ShirtSize) {
  switch (size) {
      case ShirtSize.S: console.log("small");
      case ShirtSize.M: return "medium";
      case ShirtSize.L: return "large";
      case ShirtSize.XL: return "extra large";
      // [ts] Argument of type ‘ShirtSize.XS‘ is
      // not assignable to parameter of type ‘never‘.
      default: return assertNever(size);
  }
}

prettyPrint(ShirtSize.S)
prettyPrint(ShirtSize.XXL)

In the example, we want to make sure that every time in the enum ShirtSzie has been handled by the ‘prettyPrint‘ function.

But sometime, we might miss one case. That‘s where ‘assertNever‘ function can help to make sure, we have gone though all the cases.