Five Classic Design Patterns That Actually Belong in an E-Commerce Cart

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A software developer revisited the Gang of Four Design Patterns book by building a complete e-commerce checkout flow, introducing each pattern only when a genuine requirement demanded it. Strategy handles variable shipping fees, Decorator stacks promotional rules like free shipping thresholds, and State manages the distinct behaviours of cancel or refund across order lifecycle stages. Observer decouples the payment method from downstream reactions such as emails, stock updates, and accounting, while a Facade exposes a single placeOrder() method to the controller. Two patterns were deliberately excluded, reinforcing the book's own warning against 'pattern fever': a pattern should only be applied where the flexibility it provides is genuinely needed.

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