Interface Design for Testability

Good interfaces make testing natural:

Accept dependencies, don't create them

// Testable
function processOrder(order, paymentGateway) {}

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

Return results, don't produce side effects

// Testable
function calculateDiscount(cart): Discount {}

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

Small surface area
- Fewer methods = fewer tests needed
- Fewer params = simpler test setup