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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Placing business logic directly inside an Express route handler

// Wrong: business rule trapped inside Express
app.post("/orders", (req, res) => {
  const status = req.body.total > 10000 ? "needs_review" : "approved";
});

// Correct: extracted, testable independent of Express
class Order {
  constructor(total) { this.status = total > 10000 ? "needs_review" : "approved"; }
}

The Solution //

Business logic coupled to req/res objects can only be tested by simulating an entire HTTP request, and can't be reused if the same logic is later needed from a CLI tool or a message queue consumer. Extract it into a framework-independent entity or use case that the route handler simply calls.

The Error //

Having a use case import a concrete database client directly instead of depending on an abstract interface

// Wrong: use case tightly coupled to a specific ORM
import { Order } from "./sequelize-models";
class CreateOrder { async execute(data) { return Order.create(data); } }

// Correct: depends on an abstraction, injected from outside
class CreateOrder {
  constructor(orderRepository) { this.orderRepository = orderRepository; }
}

The Solution //

This couples the use case to one specific database technology, defeating the purpose of the layering — swapping databases later requires rewriting the use case itself, not just its adapter. Depend on an abstract repository interface and inject a concrete implementation from the outer layer.