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Pascual Vila

Full-Stack Software and AI Engineer

Full-Stack Software and AI Engineer with 6 years of experience building enterprise-grade web applications across React, Angular, Node.js, and Python. Recently completed a Master's in AI Development specializing in LLMs, RAG, and AI agent architectures, and currently builds enterprise systems that integrate AI and Digital Twins to optimize industrial and logistics processes.

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Common Pitfalls & Errors

The Error //

Constructing a class's dependencies internally within its own constructor

// Wrong: hardcoded, impossible to substitute for tests
class OrderService {
  constructor() { this.repo = new PostgresOrderRepository(); }
}

// Correct: supplied from outside, swappable
class OrderService {
  constructor(repo) { this.repo = repo; }
}

The Solution //

This hardcodes the dependency permanently, making it impossible to substitute a test fake or an alternate implementation without modifying the class's own source code. Accept the dependency as a constructor parameter instead, letting the caller decide which implementation to supply.

The Error //

Using property injection (assigning a dependency after construction) instead of constructor injection

// Risky: object can be used before dependency is set
const service = new OrderService();
service.doSomething(); // BUG: repo is still undefined here!
service.repo = realRepo;

// Correct: guaranteed complete from the moment of construction
const service = new OrderService(realRepo);

The Solution //

An object using property injection can exist in an incomplete, invalid state — instantiated but with a required dependency not yet assigned — if a developer forgets the assignment step or calls a method before it happens. Constructor injection guarantees the object can never exist without its required dependencies already in place.