Software testing: the thing developers say they love in job interviews but silently curse when they have to fix a failing test at 2 AM. Fear not—this post will walk you through testing best practices, with a healthy dose of sarcasm, satire, and TypeScript examples that will make you wonder why you ever trusted npm test in the first place.

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Why Bother Testing? (a.k.a. Why Future You Will Thank Past You)

• Prevent bugs: Because debugging production at midnight is a spiritual experience only the masochists should endure.
• Confidence to change code: Refactor like a rockstar without fear of setting the server on fire.
• Better design: Turns out, writing tests forces you to think in modules instead of “one 2,000-line function should be fine.”
• Documentation: Your test is basically yelling, “Hey, this is what this function should do.”
• Review speed: Nothing says “I’m a responsible engineer” like a green checkmark.

Why not just click around manually?

Manual testing doesn’t scale. Imagine explaining to your boss that “I tested everything by opening Chrome in incognito.” Automated tests, on the other hand, are like reusable IKEA instructions—wrong sometimes, but at least they’re written down.

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The Limitations (Because Testing Is Not Magic)

Tests can’t replace humans. Exploratory testing is still a thing, because only humans can click “Checkout” 17 times in a row and discover your e-commerce app gives out free products.

So the deal is: when humans find a bug, write a test to make sure the bug never comes back. Like a zombie-proof fence.

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The Circle of Test Life

1. Write test (not 6 months later, right now).
2. Run test (regularly, not just before demos).
3. Fix test failure (quickly, not 2 weeks later with a shrug).

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Test Sizes: Small, Medium, and “Why Did I Even Run This?”

• Small test: Runs in a single process, no I/O, no sleep, no network. Basically, no fun. Use doubles for anything external.
• Medium test: Can talk to localhost, but nothing else. Sort of like inviting your friends over but not letting them use the WiFi.
• Large test: End-to-end, no rules. These validate your config and make sure the legacy system you inherited from 2008 doesn’t catch fire. Run them only in CI/CD, never in your dev loop unless you want to watch cat videos while waiting.

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Test Scope: How Much Code Are We Actually Testing?

• Unit test: One method/class. Like asking “Does my toaster toast?”
• Integration test: A couple components. Like “Does my toaster toast AND does the smoke detector panic?”
• End-to-end test: The whole system. Like “Can my toaster, smoke detector, and insurance company work together to bankrupt me?”

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The Forbidden Ice Cream Cones and Hourglasses

• Ice cream cone: All UI tests, no unit tests. Delicious, but melty and unstable.
• Hourglass: Fat integration layer, thin everywhere else. Like that one friend who skipped leg day forever.

The winning shape: The Trophy 🏆 (wide base of unit tests, fewer integration tests, a handful of end-to-end tests).

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How Not to Write a Unit Test

Bad (brittle, unclear):

test("increments value", () => {
  const counter = new Counter();
  counter._privateIncrement(); // oops, testing private API
  expect(counter.value).toBe(1); // brittle AF
});

Good (clear, stable):

test("should increment counter value when increment is called", () => {
// given
const counter = new Counter();

// when
counter.increment();

// then
expect(counter.getValue()).toBe(1);
});

Notice the given-when-then flow. It reads like natural language, not like a ransom note.

Avoiding Brittle Tests

• Test the public API, not the private internals.
• Test state, not interactions (users care about what happens, not how you do it).
• Use real objects if they’re fast and deterministic. Only mock if you absolutely have to.

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Test Doubles: The Good, the Bad, and the Mockly

• Fakes: In-memory DBs, fake queues. Great for speed.
• Stubs: Pre-programmed return values. Fine if you’re desperate.
• Mocks: Verify interactions. Useful for side-effects like “Did we send the email?” but can become your worst enemy.

Example: Stubbing Gone Wrong

const dbStub = { save: jest.fn().mockReturnValue("ok") };

test("should save data", () => {
const service = new DataService(dbStub);
service.save("data");
// brittle: relies on interaction detail
expect(dbStub.save).toHaveBeenCalledWith("data");
});

If you change your service to db.save(JSON.stringify(data)), this test breaks even though the user-facing behavior is correct. Congrats, you’ve written a brittle nightmare.

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Culture: Making Testing Cool Again™

• Orientation: Teach new hires why tests matter.
• Gamification: Internal dashboard with “Testing Levels.” Bragging rights included.
• Testing on the Toilet: Yes, literal posters in bathrooms with testing tips. Because nothing screams “company culture” like debugging mock behavior while on the throne.

Final Thoughts

• Test what matters.
• Write tests that are clear, not clever.
• Don’t chase 100% coverage like it’s the Holy Grail.
• Remember: bad tests are worse than no tests.

So go forth, write tests, and when your future self curses at you for a failing assertion, at least it’ll be a clear and meaningful one.