fenix-skills/skills/tdd/interface-design.md

Interface Design for Testability

Good interfaces make testing natural:

1. Accept dependencies, don't create them

   // Testable
   function processOrder(order, paymentGateway) {}
   
   // Hard to test
   function processOrder(order) {
     const gateway = new StripeGateway();
   }
   

2. Return results, don't produce side effects

   // Testable
   function calculateDiscount(cart): Discount {}
   
   // Hard to test
   function applyDiscount(cart): void {
     cart.total -= discount;
   }
   

3. Small surface area

   • Fewer methods = fewer tests needed
   • Fewer params = simpler test setup