Mastering code quality with SOLID principles
Mastering code quality with SOLID principles
By Dawie Pritchard, development team lead at Bluegrass Digital
At Bluegrass Digital, code quality is more than a discipline, it’s a culture. Recently, I led a practical session for our development team exploring the SOLID principles of object-oriented design. The aim was simple: to make each principle clear, show how they solve everyday coding problems, and give examples that developers can apply right away.
Why SOLID still matters in practice
When used well at real variation points, SOLID helps teams deliver faster with fewer bugs and regressions. It encourages smaller, clearer units of work, predictable extension points, and a codebase that is easier to test and evolve.
SOLID at a glance
These five principles provide a simple yet powerful framework for well-structured software.
- Single responsibility principle (SRP): one reason to change.
- Open/closed principle (OCP): open to extension, closed to modification.
- Liskov substitution principle (LSP): implementations can be swapped without breaking callers.
- Interface segregation principle (ISP): small, client-specific interfaces.
- Dependency inversion principle (DIP): high-level policy depends on abstractions, while details live behind adapters.
The real benefits on a project
Applying SOLID principles has a direct impact on the delivery process and overall software quality.
- Maintainability and fewer bugs: Small, focused modules mean lower cognitive load and simpler reviews and refactors.
- Reduced regressions and better testability: Clear seams allow parts to be tested in isolation, so changes stay local and safe.
- Flexibility and vendor independence: Depending on abstractions and using adapters keeps providers at the edges, so business logic remains untouched when changes occur.
The five principles in practice
The five principles are not abstract rules. They are a shared language for discussing good design and addressing real-world problems such as tangled dependencies and fragile code.
| Principle | What to look for | How to apply | Why it helps |
| Single responsibility principle (SRP) | Routes or components doing multiple jobs, for example validating input, writing to the database and sending email in one block. | Separate work into distinct units: validate, persist and notify. Let the controller orchestrate the flow. | Each unit has one reason to change and is simple to test. Changes do not ripple through unrelated behaviour. |
| Open/closed principle (OCP) | Editing stable code whenever a new rule appears, or growing switch statements each sprint. | Add behaviour through extension, not modification. Use middleware or decorators for cross-cutting concerns. Introduce new strategies or policy objects for domain variation. | The core remains stable while new capabilities plug in around it. Risk stays local and code reviews are simpler. |
| Liskov substitution principle (LSP) | Implementations that behave inconsistently, such as one returning a value while another throws an exception for the same condition. | Ensure implementations match the contract. If get(email) returns null when a user is missing, all implementations should behave the same rather than throw an exception. | Implementations can be swapped without touching callers. Contracts and invariants remain intact. |
| Interface segregation principle (ISP) | Functions that accept a large application context when they only use a few fields, or services that depend on an entire framework when they only read data. | Split broad interfaces into smaller, role-specific ones. Depend on UserReader, UserWriter or UserAuthenticator instead of a large, catch-all context. | Reduces coupling and clarifies dependencies. Makes tests smaller, faster and easier to maintain. |
| Dependency inversion principle (DIP) | Business logic calling vendor SDKs directly, or using vendor configuration objects in the core. | Define domain ports such as Payments or Mailer. Place SDKs behind adapters at the boundary and wire them in a composition root. Swapping providers should not affect business rules. | The domain depends on abstractions, not vendors. Providers can be changed without rewriting business logic or tests. |
Applying SOLID in modern JavaScript and TypeScript
These principles are just as relevant in today’s JavaScript and TypeScript projects. Model contracts using interfaces or types, and prefer composition over inheritance. Keep business rules framework-agnostic and treat frameworks and SDKs as adapters at the edges. Use a composition root where ports are wired to adapters, starting simple with constructors or factories, and only introducing a dependency injection container when object graphs become complex.
Bluegrass’s commitment to code quality
At Bluegrass Digital, SOLID is not a buzzword, it is part of our engineering habits. We invest in regular technical sessions, shared learning and structured professional development to ensure these habits take root across teams. This approach helps keep our projects safe to change, efficient to maintain and dependable for clients.
Closing thoughts
SOLID works best when it guides everyday decisions. Name the principle, apply the smallest change that honours it, and keep moving. Split responsibilities, extend rather than edit, honour contracts, depend only on what you genuinely use, and push vendor details to the edges. That is how Bluegrass Digital keeps change cheap, safe and predictable.
