🧱 TypeScript — Generic Classes: Build Reusable and Type-Safe Object-Oriented Structures

🧲 Introduction – What Are Generic Classes in TypeScript?

In TypeScript, generic classes allow you to define class templates that work with multiple data types. By using generics, you can build flexible, type-safe, and reusable class structures—ideal for managing data models, services, utilities, and data storage logic. Rather than writing separate classes for each type, generic classes help you define one blueprint that adapts to any type passed at instantiation.

🎯 In this guide, you’ll learn:

• What generic classes are and how to declare them
• How to use type parameters in properties and methods
• Practical use cases for generic classes
• Best practices and common pitfalls

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🧾 What Is a Generic Class?

A generic class in TypeScript uses a type parameter (like <T>) to indicate that it can work with any type. The type is specified when the class is instantiated.

✅ Basic Syntax:

class Container<T> {
  private value: T;

  constructor(val: T) {
    this.value = val;
  }

  getValue(): T {
    return this.value;
  }
}

✅ Usage:

const stringBox = new Container<string>("TypeScript");
console.log(stringBox.getValue()); // Output: TypeScript

const numberBox = new Container<number>(123);
console.log(numberBox.getValue()); // Output: 123

📌 The same class works for string, number, or any custom type, depending on how you instantiate it.

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🔄 Generic Class with Multiple Type Parameters

You can define a class with two or more type parameters.

class Pair<K, V> {
  constructor(public key: K, public value: V) {}

  display(): void {
    console.log(`${this.key}: ${this.value}`);
  }
}

const pair = new Pair<string, number>("id", 101);
pair.display(); // Output: id: 101

✅ This is commonly used in key-value pairs, maps, and generic containers.

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🧩 Generic Classes with Constraints

You can restrict the allowed types using extends to create generic constraints.

class Logger<T extends { toString(): string }> {
  log(value: T): void {
    console.log("Logged:", value.toString());
  }
}

const logger = new Logger<Date>();
logger.log(new Date()); // ✅ Works because Date has a toString method

📌 This ensures only types that match the constraint can be used with the class.

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🧠 Generic Class Example – Data Repository

A typical use case is creating a generic repository for handling data.

class Repository<T> {
  private items: T[] = [];

  add(item: T): void {
    this.items.push(item);
  }

  getAll(): T[] {
    return this.items;
  }
}

interface User {
  name: string;
  email: string;
}

const userRepo = new Repository<User>();
userRepo.add({ name: "Alice", email: "alice@example.com" });
console.log(userRepo.getAll());

✅ Generic repositories like this are used in CRUD operations, data stores, and caching layers.

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🔍 Real-World Use Cases for Generic Classes

• ✅ API clients returning different data models
• ✅ Data storage for typed entities
• ✅ Queue or stack implementations
• ✅ Form field validation handlers
• ✅ Reusable utility services that work with any type

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⚠️ Common Mistakes and How to Avoid Them

❌ Mistake	✅ Solution
Omitting type when instantiating the class	Always specify the type parameter (e.g., new Container<string>())
Overcomplicating with too many type params	Keep generics focused and clear; only add what’s necessary
Not using constraints when needed	Use extends to ensure the generic type meets certain requirements
Confusing type inference in methods	Explicitly annotate return and parameter types where inference fails

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💡 Best Practices for Generic Classes

• ✅ Name type parameters clearly (e.g., T, K, V, R)
• ✅ Use constraints to restrict accepted types when necessary
• ✅ Prefer composition over inheritance if flexibility becomes limited
• ✅ Keep classes simple and focused on a single responsibility
• ✅ Combine with interfaces to create extensible systems

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📌 Summary – Recap & Key Takeaways

Generic classes in TypeScript offer a scalable and type-safe way to write reusable object-oriented code. Whether you’re building data handlers, utility services, or components, generics ensure that your logic stays consistent while adapting to different types.

🔍 Key Takeaways:

• Generic classes use <T> (or more) to support multiple types
• Enable strong type-checking and reusability
• Support constraints via extends to enforce structure
• Widely used in repositories, storage, services, and utility components

⚙️ Practical relevance: Generic classes are foundational in enterprise APIs, design patterns, data layers, UI frameworks, and backend services.

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❓ FAQs – Generic Classes in TypeScript

❓ Can classes in TypeScript be generic?
✅ Yes. Use <T> to define generic type parameters in class declarations.

❓ What is the purpose of generic classes?
📌 They allow you to reuse the same class logic for multiple types while maintaining type safety.

❓ Can I use multiple type parameters in a class?
✅ Yes. You can use <T, U>, <K, V>, etc., for multiple generics.

❓ Are constraints supported in generic classes?
✅ Absolutely. You can restrict generic types using extends.

❓ Should I always use generics in classes?
❌ No. Use generics when your class logic depends on the type passed. Avoid unnecessary abstraction.

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