Introduction to Object-Oriented Design (OOD)
Object-Oriented Design (OOD) is a critical skill for modern software engineers, particularly in the context of Designing Low-Level Systems (LLD). While Object-Oriented Programming (OOP) focuses on the language mechanics (classes, objects, inheritance), OOD is the art of using these tools to create flexible, maintainable, and scalable software architectures.
What is an OOD Interview?
In an OOD interview (often called a Low-Level Design interview), candidates are asked to design a localized system—like a Parking Lot, an Elevator System, or a Vending Machine.
Unlike System Design interviews, which focus on high-level scalability (databases, load balancers, microservices), OOD interviews focus on:
- Class Structure: How should objects interact?
- Relationship Modeling: Using Composition, Aggregation, and Inheritance correctly.
- Design Patterns: Applying patterns like Singleton, Factory, or Strategy to solve recurring problems.
- SOLID Principles: Ensuring the design is robust and easy to extend.
Why OOD Matters for JavaScript Developers
In modern JavaScript/TypeScript development (especially with Node.js and frameworks like NestJS), complex business logic is often organized into classes and services. Understanding OOD helps you:
- Manage Complexity: Break down large problems into manageable objects.
- Improve Testability: Decoupling components makes unit testing much easier.
- Enhance Collaboration: Shared design patterns provide a common vocabulary for teams.
OOD vs. OOP: The Difference
| Concept | OOP (Programming) | OOD (Design) |
|---|---|---|
| Focus | Implementation and syntax. | Logic, structure, and abstraction. |
| Goal | Creating objects and reusing code. | Organizing objects for flexibility. |
| Tools | Classes, Interfaces, Prototype. | Design Patterns, UML, SOLID. |
Core Principles to Master
Before diving into specific design problems, you must be comfortable with the following foundations:
- SOLID Principles: Five design principles that make software more understandable and flexible.
- DRY (Don't Repeat Yourself): Minimizing redundancy.
- KISS (Keep It Simple, Stupid): Avoiding over-engineering.
- Composition over Inheritance: Preferring building complex objects from smaller ones rather than creating deep inheritance trees.
Copyright & Originality
All content in this Object-Oriented Design (OOD) curriculum, including the explanations, TypeScript implementations, and Mermaid diagrams, has been authored specifically for this resource.
- Original Synthesis: The designs are synthesized from common industry architectural patterns and standard interview tropes (like the "Parking Lot" or "ATM").
- No Plagiarism: The content is unique and does not copy the specific phrasing, structure, or visual assets of external resources such as ByteByteGo, Educative, or GeeksForGeeks.
- Code-First Approach: Unlike many high-level design resources, our focus is on Low-Level Design (LLD) with production-ready TypeScript code, making it a distinct and original learning tool.
In the next section, we will explore a step-by-step framework to tackle any OOD problem during an interview or in a real-world project.