Lesson 05 — Advanced types QA relies on

Title: Enums, void, unknown, never — and why any is risky

Description: Pick the right tool when modeling statuses, logging helpers, uncertain JSON, or impossible branches TypeScript must narrow away.

Why it matters for QA: Reports (Passed, Skipped), flaky classifications, API probes — expressive types communicate intent inside suites.

Type cheat sheet

1. enum for fixed sets

enum TestStatus {
  Passed,
  Failed,
  Skipped,
}

enum Role {
  Admin,
  User,
  Guest,
}

const lastRun: TestStatus = TestStatus.Passed;

String enums are common in web services — learn both styles as you read existing codebases.

2. void for side-effect utilities

function logStep(message: string): void {
  console.log(`[STEP] ${message}`);
}

3. Avoid any

// Discouraged — everything compiles, nothing is trustworthy.
let data: any = "hello";
data = 123;

During greenfield QA work prefer unknown + guards or explicit DTO interfaces.

4. unknown stays safe until narrowed

let response: unknown = "from network";

if (typeof response === "string") {
  console.log(response.toUpperCase());
}

5. never for deliberate throws

function throwError(message: string): never {
  throw new Error(`${JSON.stringify(message)}`);
}

6. Practice

1. Replace any with unknown

Instead of using any for a fixture parser, use unknown. Then:

• check the type using typeof
• check arrays using Array.isArray()

👉 This makes the code safer, because you must validate data before using it.