As you build JavaScript applications, you will encounter a variety of problems that can be solved more efficiently and elegantly with design patterns. Understanding which pattern to apply depends on the specific challenge you face.

• If you find yourself needing to create a single, shared instance of an object, the Singleton pattern is ideal;
• When you need to create objects that share a common interface but may differ in their internal behavior, the Factory pattern helps you centralize object creation logic;
• If your codebase is growing and you want to encapsulate related functionality while keeping the global scope clean, the Module pattern is a strong choice;
• When you want to dynamically add responsibilities to objects without modifying their structure, the Decorator pattern can enhance flexibility.

If you notice that your code is becoming cluttered with complex interfaces, the Facade pattern can help by providing a simpler interface to a set of interfaces in a subsystem. When you need to control access to certain objects, perhaps to add validation or logging, the Proxy pattern is effective. For situations where you want to notify multiple parts of your application about changes to a particular object, the Observer pattern is invaluable. The Command pattern is perfect when you need to encapsulate requests as objects, allowing you to queue, log, or undo operations. Finally, if you want to enable an object to change its behavior based on its internal state or external context, the Strategy pattern allows you to swap algorithms easily.

Recognizing when to refactor your code using a design pattern is a crucial skill. If you see repeated code structures or find that making changes introduces bugs in multiple places, it might be time to introduce a pattern. When your code becomes difficult to maintain, extend, or test, design patterns can provide a roadmap for improvement. Pay attention to warning signs such as duplicated logic, tight coupling between modules, or classes that try to do too much. These are strong indicators that applying a design pattern could simplify your code and make it more robust.