Safe JSON Decoder

type DecodeResult<T> =
  | { ok: true; value: T }
  | { ok: false; error: string };

interface CourseProgress {
  course: string;
  completedLessons: number;
}

function isRecord(value: unknown): value is Record<string, unknown> {
  return typeof value === "object" && value !== null;
}

function decodeCourseProgress(json: string): DecodeResult<CourseProgress> {
  let parsed: unknown;

  try {
    parsed = JSON.parse(json) as unknown;
  } catch (error: unknown) {
    return { ok: false, error: error instanceof Error ? error.message : "Invalid JSON" };
  }

  if (!isRecord(parsed)) {
    return { ok: false, error: "Expected an object" };
  }

  if (typeof parsed.course !== "string" || typeof parsed.completedLessons !== "number") {
    return { ok: false, error: "Invalid progress shape" };
  }

  return {
    ok: true,
    value: {
      course: parsed.course,
      completedLessons: parsed.completedLessons,
    },
  };
}

const result = decodeCourseProgress('{"course":"typescript","completedLessons":10}');
console.log(result);