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tdd/interface-design.md

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+# Interface Design for Testability
+
+Good interfaces make testing natural:
+
+1. **Accept dependencies, don't create them**
+
+   ```typescript
+   // Testable
+   function processOrder(order, paymentGateway) {}
+
+   // Hard to test
+   function processOrder(order) {
+     const gateway = new StripeGateway();
+   }
+   ```
+
+2. **Return results, don't produce side effects**
+
+   ```typescript
+   // 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