[danvk]

If you prefer the "or throw" approach to handling null values, you can write a little helper to get something like it in TypeScript:

    function assert<T>(v: T | null | undefined, message?: string): asserts v is T {
      if (v === null || v === undefined) {
        throw new Error(message);
      }
    }

    function parseFile(file: File | null) {
      assert(file, 'File must exist');
      // TS now infers type of file as File
    }

Check out the TypeScript 3.7 release notes for more on assertion functions: https://www.typescriptlang.org/docs/handbook/release-notes/t...