When I started my career in software development seven years ago, I was all about writing code that worked. That moment when a feature finally clicked into place after hours of trial-and-error felt like magic. But over the years—through code reviews, production incidents, and scaling systems across teams—I’ve come to realize that functional code is just the surface. The real craft lies in writing code that is maintainable, scalable, and understandable by others.

Enter: the SOLID principles.

These five time-tested guidelines have helped me design better systems, lead cleaner projects, and mentor junior developers across codebases built in TypeScript and Swift alike. In a world full of frameworks, fast releases, and shifting requirements, SOLID has stayed relevant—not because it's trendy, but because it simply works.

Let me walk you through how I use these principles every day, grounded in practical examples from both frontend TypeScript development and mobile apps built with Swift.

Single Responsibility Principle

This principle clicked for me early on, when I inherited a TypeScript service that handled authentication, logging, email notifications, and user session management—all in one file. Every time a change was needed, there was a real risk of breaking something unrelated. The cognitive load was massive.

Now, whether I’m working on a frontend React app or a Swift-based iOS app, I push for strict separation. Each class or module should have one, and only one, reason to change. In TypeScript, this often looks like splitting components, hooks, and utility functions by responsibility. In Swift, I apply the same mindset to ViewModels and Use Cases: UI logic and business logic should never mix.

When each piece does just one thing, refactoring becomes a joy, not a risk.

Open/Closed Principle

I really started appreciating OCP during a social login integration. We initially supported only Google sign-in, so the logic was tightly embedded in the app. But once we needed to add Apple and Facebook logins, modifying the core logic quickly became painful.

Today, I design for extension from the beginning—especially in TypeScript, where interfaces allow us to define contracts that each login method implements. The main logic doesn’t care how the user authenticates—it just calls the expected interface. In Swift, using protocols and extensions makes this equally clean—new providers can be introduced without modifying the core flow.

This principle is the antidote to “if-else” hell and gives your code a plug-and-play feel.

Liskov Substitution Principle

LSP is one of those principles that you don’t fully understand until something breaks.

In Swift, I once created a subclass that overrode a function to do nothing, thinking it was harmless. Turns out, some parts of the code relied on that function to perform a side-effect. That’s when I truly understood LSP: if you can’t substitute a subclass without altering the behavior clients expect, your inheritance model is broken.

In TypeScript, the same applies—especially when you’re extending base components or abstract services. Violating LSP leads to subtle, frustrating bugs. These days, I lean more toward composition and interfaces to avoid this entirely.

Interface Segregation Principle

This one really hits home when you're dealing with large, shared interfaces across teams.

I’ve worked on TypeScript projects where a single interface had a dozen methods, most of which were irrelevant to 80% of its consumers. Updating one method would cause ripple effects in parts of the codebase that didn’t even use it.

The fix? Break interfaces down into smaller, role-focused units. A UI component shouldn’t care about admin-only logic. Similarly in Swift, I design protocols to be minimal and precise—exposing only what each consumer actually needs. Smaller, focused protocols lead to cleaner implementations and fewer accidental bugs.

Dependency Inversion Principle

In one of my earlier TypeScript projects, our logger was directly wired to a third-party library. When we decided to swap it out, it was a nightmare. That was my first real lesson in DIP: high-level modules should not depend on low-level details.

Now, I always build against abstractions—custom interfaces in TypeScript and protocols in Swift—and inject dependencies at runtime. Whether it’s a logger, analytics tool, or API client, this pattern makes your codebase resilient to change and easy to test.

In Swift, this aligns perfectly with dependency injection via initializers or lightweight DI containers. You can swap out real services with mocks or stubs during testing without touching the actual logic.

Why It Still Matters in 2025

With everything moving so fast—AI tooling, edge computing, cloud-first architectures—it’s tempting to think we can skip the basics. But the truth is, these fundamentals are more important than ever.

As a tech lead, I don’t look for fancy patterns or clever hacks. I look for code that respects boundaries, honors contracts, and is predictable under pressure. SOLID gives us a shared language for building good software—whether it’s a TypeScript-based frontend, a SwiftUI app, or something in between.

These principles:

• Keep teams aligned
• Make onboarding easier
• Reduce bugs
• Future-proof your codebase

Final Thoughts

I didn’t learn SOLID from books—I learned it through debugging spaghetti code at 2 AM, refactoring brittle systems, and watching junior devs struggle with the same problems I once faced.

If you’re starting out, pick one principle and try applying it in your next project. If you’re mid-level, start recognizing violations and proactively cleaning them up. And if you’re leading teams, teach these principles through code reviews, design discussions, and mentorship.

Because at the end of the day, good code isn’t just what works—it’s what lasts.