SOLID Principles – MicroSaaS Development Guideline
SOLID is a set of five principles for writing clean, scalable, and maintainable code — especially useful in fast-moving SaaS projects where code clarity and modularity matter.
These guidelines are written with JavaScript/TypeScript (Next.js) in mind, but they apply across any modern backend/frontend architecture.
🔁 Overview of SOLID
Principle Description
S – Single Responsibility A class/module/function should have one reason to change
O – Open/Closed Code should be open for extension, closed for modification
L – Liskov Substitution Subtypes must be substitutable for their base types
I – Interface Segregation Prefer small, specific interfaces over large, general ones
D – Dependency Inversion Depend on abstractions, not concrete implementations
✅ How to Apply SOLID in Practice
S – Single Responsibility Principle
"Do one thing, and do it well."
Break components, functions, and services into focused units.
Example: Separate API service logic from UI rendering logic in a page.
🔧 In Next.js: Keep API routes (/api/) focused on handling a single resource per file.
O – Open/Closed Principle
"You should be able to add features without modifying existing code."
Use abstraction to enable plug-and-play behavior.
Favor extending components over editing existing ones.
🔧 Example: Build a component that accepts props for styling/behavior instead of hardcoding different versions.
L – Liskov Substitution Principle
"If A is a subtype of B, you should be able to use A anywhere you use B."
Ensure extended classes or components don’t break base contracts.
🔧 Example: A custom FormInput should behave like a standard input (accept value, onChange, etc.).
I – Interface Segregation Principle
"Keep interfaces lean and focused."
Avoid bloated prop types or interfaces.
Create multiple small types instead of one big UserInterface.
🔧 In TypeScript: Use Pick, Omit, or composition (&) to build targeted interfaces.
D – Dependency Inversion Principle
"High-level modules shouldn’t depend on low-level modules. Both should depend on abstractions."
Use dependency injection where possible.
Separate business logic into services or hooks, and inject them into UI components.
🔧 Example: Pass an authService to a page rather than hardcoding Supabase calls inside the page logic.
🔍 Tips for Staying SOLID in JavaScript/TypeScript
Use interface and type extensively in TypeScript
Abstract shared logic into hooks (useXyz) and services (authService, billingService)
Avoid deep nesting and large files — break code into clear layers
Test each part in isolation (unit testing aligns perfectly with SOLID)
🧪 Recommended Patterns
Component per responsibility → , ,
Hooks for logic → useUser, useSubscription, useFeatureFlags
Services for backend calls → userService.get(), billingService.subscribe()
Abstractions for third-party APIs → Wrap Stripe, Supabase, or Sendgrid behind interfaces