Software Design Patterns

Introduction

• Junior developers' code often likened to Play-Doh snakes.
• Senior developers' code compared to the Sistine Chapel.
• Principal engineers revert to simple patterns for maintainability.
• Lecture will cover 10 software design patterns, including pros and cons.
• Patterns categorized:
  • Creational (object creation)
  • Structural (object relationships)
  • Behavioral (object communication)

Importance of Design Patterns

• Influential text: Design Patterns by the Gang of Four.
• Design patterns help solve recurring problems in programming.
• Key to being a proficient software engineer: problem-solving, not syntax memorization.

Singleton Pattern

• Only one instance of an object can be created.
• JavaScript example: global object vs singleton class.
• Use language features before patterns to avoid unnecessary boilerplate.

Prototype Pattern

• Alternative to inheritance: clones objects instead of creating class hierarchies.
• JavaScript supports prototypal inheritance natively.
• Use Object.create to set up inheritance from a prototype object.

Builder Pattern

• Construct objects step-by-step using methods instead of constructors.
• Useful for managing complex object creation processes.
• Method chaining allows for the accumulation of settings/options.

Factory Pattern

• Uses methods to instantiate objects, avoiding the new keyword.
• Ideal for cross-platform apps with different implementations for iOS and Android.

Facade Pattern

• Simplifies complex systems by providing a simple API.
• Example: Simplifying electrical and plumbing operations in a house with a single interface.
• Common in many JavaScript packages (e.g., jQuery).

Proxy Pattern

• Acts as a substitute for another object.
• Example: Vue.js reactivity system uses proxies to intercept data changes and update the UI.
• Useful for large objects to simulate the original without duplicating.

Iterator Pattern

• Traverses collections of objects.
• Custom iterator in JavaScript can implement a range function with next and done properties.
• Supports loops like for...of using symbol iterator.

Observer Pattern

• One-to-many relationship: multiple objects subscribe to events from a single object.
• Used in reactive systems like Firebase and RxJS for real-time updates.
• Subject class in RxJS allows multiple subscriptions and broadcast changes.

Mediator Pattern

• Manages complex communication between many objects.
• Example: Air traffic controller managing airplane and runway interactions.
• Middleware in Express.js as mediator for HTTP requests and responses.

State Pattern

• Represents an object's behavior based on its state.
• Avoids complex conditional logic by using state-specific classes.
• Example: Different behaviors based on an object's mood/state using finite state machines.

Conclusion

• Design patterns can significantly simplify complex code and make it more maintainable.
• Practical real-world implementations illustrate the utility of each pattern.
• Further learning recommended on additional design patterns.