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Darwin Manalo
Darwin Manalo

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Test First or Regret Later: The Pros and Cons of TDD

#tdd #javascript #node #jest

Let’s be honest—when I first heard about Test-Driven Development (TDD), my initial reaction was something like: “Wait, you want me to write tests… before I even write the code? That’s like writing a movie review before watching the film.” It felt backward, overly rigid, and honestly, like a productivity killer.

But, as with many things in tech, what started as skepticism eventually turned into something close to obsession.

These days, TDD has become a core part of my development workflow—something I reach for not because I have to, but because it genuinely helps me write better, cleaner, more maintainable code. In this post, I want to share my personal (and slightly opinionated) take on why TDD has earned a permanent spot in my toolbox.

📚 Table of Contents

It’s hard to commit to something when you don’t know what it’s for, why it matters, or if it’s even worth your time. That’s exactly how I felt about TDD—until I gave it a real shot.

So here are some of the biggest reasons why I keep reaching for TDD every time I build something serious:

1. It Forces Your Functions to Be Better—Like, Actually Better

One of the coolest (and low-key most frustrating at first) things about TDD is that it forces you to write better functions. Cleaner. More independent. Less clingy. If you've heard of Dependency Injection, this is where it starts to make sense.

Let me explain with some simple examples.

Example 1:

Without Dependency Injection (a tightly coupled mess):

function getUserProfile(userId) {
  const db = new Database(); // hardcoded dependency
  return db.findUserById(userId);
}
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Looks fine, right? But if you want to test getUserProfile, you now have to deal with a real Database object—or mock it in weird ways. Yikes.

With Dependency Injection (TDD-friendly):

function getUserProfile(userId, db) {
  return db.findUserById(userId);
}
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Now, your function is no longer responsible for creating the database—it just uses it. This makes it super easy to test:

// In your test
const mockDb = {
  findUserById: jest.fn().mockReturnValue({ id: 1, name: "Test User" })
};

const result = getUserProfile(1, mockDb);

expect(result.name).toBe("Test User");
expect(mockDb.findUserById).toHaveBeenCalledWith(1);
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See? TDD gently pushes (okay, maybe shoves) you toward writing functions that don’t rely on global state or hidden dependencies. And once you get used to it, you’ll start doing it even when you're not writing tests first.

Example 2:

Without Dependency Injection:

function sendWelcomeEmail(userEmail) {
  const emailService = new EmailService();
  emailService.send(userEmail, "Welcome!", "Thanks for joining us!");
}
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Now you’ve got a problem: every time you test this function, it might actually send an email (or crash if EmailService isn’t mocked right). Not fun.

With Dependency Injection:

function sendWelcomeEmail(userEmail, emailService) {
  emailService.send(userEmail, "Welcome!", "Thanks for joining us!");
}
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In your test:

const mockEmailService = {
  send: jest.fn()
};

sendWelcomeEmail("test@example.com", mockEmailService);

expect(mockEmailService.send).toHaveBeenCalledWith(
  "test@example.com",
  "Welcome!",
  "Thanks for joining us!"
);
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Now you're not sending real emails—you’re just verifying the logic. Clean and safe.

2. Refactoring Without Fear

You ever try to clean up some messy code and suddenly everything breaks? You move one tiny block, and the whole tower crashes. Refactoring without tests is like operating without a safety harness—you’re just hoping for the best.

But with TDD? You can refactor like a boss.

Since your tests already cover the expected behavior, you’re free to improve variable names, break functions apart, swap implementations, or go full-on refactor ninja—without the “what if I broke something?” anxiety.

Let’s say you have this working but ugly function:

function isValidUser(user) {
  return user && user.name && user.email && user.email.includes("@");
}
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You decide to clean it up:

function isValidUser(user) {
  if (!user) return false;
  const { name, email } = user;
  return Boolean(name && email && email.includes("@"));
}
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If you’ve got tests like:

test("returns true for valid user", () => {
  expect(isValidUser({ name: "Sam", email: "sam@example.com" })).toBe(true);
});

test("returns false for missing email", () => {
  expect(isValidUser({ name: "Sam" })).toBe(false);
});
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You can refactor without holding your breath. If the tests pass, you’re good. If they fail, they’ll tell you exactly what broke. It’s like having a personal safety net made of logic and assertions.

Bonus Scenario: When Someone Else Breaks Your Code

Here’s a spicy one—ever had a teammate “clean up” your utility function and accidentally break three other things that depended on it? Yeah, me too.

Let’s say you wrote a humble little function:

function formatUsername(user) {
  return `@${user.name.toLowerCase()}`;
}
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You’ve got tests:

test("formats username with @ and lowercase", () => {
  expect(formatUsername({ name: "Alice" })).toBe("@alice");
});
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All good. But then someone comes in and decides to improve it:

function formatUsername(user) {
  return user.name.startsWith("@") ? user.name : `@${user.name}`;
}
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Seems harmless, right? But now toLowerCase() is gone, and other parts of the app relying on lowercase usernames start to break. Users can’t log in, the display gets inconsistent—chaos!

But because of your test, the break is caught immediately:

FAIL: formats username with @ and lowercase
Expected: "@alice"
Received: "@Alice"
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Boom. Now your teammate knows exactly what they broke, and where. No drama. No guessing. No 3 a.m. debugging.

This is the magic of TDD: it protects your code from future “well-meaning” edits, whether they’re by you or someone else.

3. Your Tests Double as Documentation (Seriously)

One underrated superpower of TDD? Your tests become living documentation.

Ever joined a project where you had no idea how a utility function works, and the only option was to run the whole app or spam Postman requests just to figure it out? Yeah, nightmare fuel. But with a solid test suite, you can just open the test file and boom—you get a step-by-step guide on how the function is supposed to behave.

Example:

Instead of guessing what this does:

calculateTotal([{ price: 100 }, { price: 50 }], 0.1);
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You can just look at the test:

test("calculates total with 10% discount", () => {
  const items = [{ price: 100 }, { price: 50 }];
  const discount = 0.1;

  const result = calculateTotal(items, discount);

  expect(result).toBe(135);
});
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That’s basically a how-to. It tells you what the function expects, what it returns, and under what conditions. You can even run the tests to play around with scenarios and see how things respond—no UI, no API, just your logic doing its thing in isolation.

This is huge for onboarding, debugging, and even writing new features that depend on existing code. With good tests, your functions tell their own story.

4. Maintainability Goes Through the Roof (Hello, Enterprise Projects)

If you're building something small, you might get away with spaghetti code and no tests (not recommended, but hey, we’ve all done it). But in enterprise projects—where the codebase grows faster than your caffeine tolerance—maintainability becomes everything.

TDD naturally leads to code that’s easier to maintain, refactor, and extend. Why? Because everything is written with testability in mind. Functions are decoupled. Side effects are controlled. You’re thinking ahead instead of duct-taping fixes.

And when that giant enterprise app has 300+ contributors, changing one line of logic doesn’t have to feel like defusing a bomb. The tests will tell you immediately if something downstream breaks. It gives teams confidence to move fast without fear—which is gold in large-scale development.

TDD turns your codebase from a fragile castle of cards into something that can actually survive growth, turnover, and scale.

5. No More “Oops, That Bug Made It to Production” Moments

Let’s be real: developers forget things. Tests don’t.

Even if someone on the team skips running tests locally (ahem, we see you), having tests integrated into your CI/CD pipeline acts like an automated gatekeeper. Any commit or pull request that breaks a test? Boom—pipeline fails. The code doesn’t get deployed. Simple as that.

This is where TDD really shines. Because your tests are written first, they’re always ready to catch regressions. They’re not an afterthought—they’re part of the design.

So even if your teammate forgets to run npm test, your CI setup won’t. And that means fewer bugs slipping into production, fewer rollback nightmares, and more time spent building cool stuff instead of putting out fires.

6. Code Coverage Keeps Everyone Honest

Let’s be honest—just because someone says they wrote tests doesn’t always mean they tested everything that matters. That’s where code coverage steps in.

By setting up a minimum test coverage threshold (say, 80% or higher) in your CI/CD pipeline, you’re not just checking if tests exist—you’re checking if they actually run through your code. No more sneaky commits with “tests” that don’t touch the real logic.

Example:
You can configure tools like Jest in JavaScript to fail builds if coverage drops below a certain percentage:

--coverage --coverageThreshold='{"global": {"branches": 80,"functions": 80,"lines": 80,"statements": 80}}'
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This ensures that your TDD discipline doesn’t go to waste. It’s like telling your team: “If you wrote a function, you better show us it works.”

Code coverage isn’t about hitting 100% just for bragging rights. It’s about visibility, accountability, and keeping your codebase clean—even in a fast-moving team or enterprise environment.

But TDD Isn’t Perfect

I know I’ve been hyping TDD like it’s the best thing since dark mode—but let’s be honest, it’s not all smooth sailing. Like any good tool or practice, it comes with trade-offs.

Because loving something means being able to call out its flaws too, right? These are the moments where TDD can feel like a headache instead of a hero.

1. TDD Has a Steep Learning Curve for Beginners

If you’re just starting out in programming, TDD can feel like learning to drive with a manual transmission—while also solving a Rubik’s cube.

You’re trying to wrap your head around functions, logic, syntax—and now you also have to write tests for code you haven’t even written yet? It’s a lot.

For beginners, TDD might feel like a roadblock instead of a learning aid. The mental shift it requires—thinking in small, testable steps—comes with time and experience. Without the right guidance, it can feel more like a chore than a benefit.

But here’s the twist: once you push through the discomfort, TDD can actually accelerate learning. It teaches you to think about code behavior, write modular functions, and debug with more clarity. It's just… not exactly beginner-friendly right out of the gate.

2. It Slows You Down at First (And That Can Be Frustrating)

When you're new to TDD, it feels like you're coding in slow motion. Write a test. Watch it fail. Write just enough code. Run it again. Refactor. Repeat. Meanwhile, your friend just finished the whole feature in half the time (but also introduced 3 bugs).

TDD is an investment. It slows you down now to save you time (and stress) later. But it does require patience.

3. Not Everything Is Easy to Test

Sometimes you’re working with things that are just… awkward to test. Think file uploads, external APIs, real-time WebSockets, or stuff that touches hardware. You can mock, stub, and isolate all you want, but some scenarios still feel clunky.

TDD doesn't always play nicely with messy edge cases or complex dependencies unless you put in extra effort to architect them for testability.

4. It’s Easy to Fall Into “Test Hell”

Bad tests are worse than no tests. If your tests are fragile, overly specific, or tightly coupled to implementation details, they break every time you refactor—even if the actual behavior didn’t change.

This leads to devs muttering “why are we even doing this?” and turning off the test suite. Moral of the story: write meaningful tests, not just test coverage filler.

5. It Doesn’t Replace Real QA

TDD catches logic bugs and regressions, but it won’t save you from UI/UX fails, accessibility issues, or confusing user flows. It's not a silver bullet. You still need real users (or QA folks) clicking around and breaking things in creative ways.

6. Can Be Overkill for Tiny Scripts or Throwaway Code

Not everything needs full-blown TDD. A one-off script or small personal tool? Writing tests for every line might be more work than it’s worth. Use your judgment—it’s okay to break the rules when it makes sense.

7. Clients With Tight Budgets Won’t Always Love TDD

Let’s be real—when you’re working with a client who’s on a shoestring budget and wants the app done yesterday, explaining the value of writing tests first can feel like asking them to pay extra for invisible features.

They don’t always see the long-term payoff of TDD. They want buttons that work, screens that load, and a launch ASAP. Writing tests upfront might sound like you’re stalling—or worse, billing for “nothing.”

In these situations, you have to balance pragmatism and principle. Sometimes it makes sense to skip TDD (or scale it down), especially for MVPs or prototypes. Just be honest with yourself: skipping tests now means you’re betting on fixing things later. And later always costs more.

🧠 Final Thoughts

Test-Driven Development isn’t magic. It won’t write your code for you or stop every bug in its tracks. But it does change the way you think, structure, and maintain your code—for the better.

Yes, it can be frustrating. Yes, it takes time to learn. And yes, sometimes you just want to hack something together and skip the tests. I get it. But in my experience, the benefits of TDD almost always outweigh the friction.

It’s not about writing perfect tests—it's about building code that’s easier to trust, change, and grow. Whether you're working solo, in a fast-moving team, or scaling a giant enterprise app, TDD can be the quiet hero that keeps your codebase from imploding.

So if you’re still on the fence, give it a real shot. Start small. Break things. Test things. You might just end up liking it.

💬 What About You?

Tried TDD yet? Still pretending your code “works on my machine”? Or maybe you’ve mastered the ancient art of commenting out console.log? 😏

Whether you swear by tests or swear at them, I’d love to hear your take. Drop a comment below—rant, cheer, confess.

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