interface-design

name	interface-design
description	Interface design for testability (dependency injection, pure results, small surface). Use when designing APIs, reviewing testability, or refactoring for tests.

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