🌟 Understanding Generics in TypeScript: The Key to Reusable, Type-Safe Code

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Generics are a fundamental feature in TypeScript that allow developers to create reusable, flexible, and type-safe components.
They enable functions, classes, and interfaces to work with different data types without losing type information — ensuring both code reusability and type safety.

🧩 Why Do We Need Generics?

1. Code Reusability

Generics allow you to write a single function, class, or interface that can operate on different data types.

Without generics, you might end up writing multiple versions of the same function for each data type.
Generics solve that problem by using type parameters — placeholders for any type. 

function identity<T>(arg: T): T {
  return arg;
}

let output1 = identity<string>("myString");
let output2 = identity<number>(100);

Here, <T> acts as a type variable, allowing the same function to handle both string and number without duplicating logic.

2. Type Safety

Generics enable compile-time type checking, catching errors early in the development process — long before they cause runtime issues.

TypeScript knows exactly what type you’re working with, which improves both reliability and maintainability.

3. Improved Code Readability

By explicitly defining generic types, you make your code’s intent clearer.
Readers and collaborators can easily understand the expected data types and structure.

🧠 Generic Building Blocks

Type Parameters

Generic types use type parameters, usually represented by uppercase letters inside <>:

Symbol Meaning
T Type
K Key
V Value

⚙️ Generic Functions

A generic function allows you to handle multiple data types with one implementation. 

function identity<T>(arg: T): T {
  return arg;
}

let output1 = identity<string>("Srashti");
let output2 = identity<number>(25);

🧾 Generic Interfaces

Generic interfaces define contracts for objects that can work with multiple data types. 

interface GenericIdentityFn<T> {
  (arg: T): T;
}

function identity<T>(arg: T): T {
  return arg;
}

let myIdentity: GenericIdentityFn<number> = identity;

This ensures that the identity function only works with the data type defined (number in this case).

🏗️ Generic Classes

A generic class can handle data of different types dynamically. 

class GenericNumber<T> {
  zeroValue: T;
  add: (x: T, y: T) => T;
}

let myGenericNumber = new GenericNumber<number>();
myGenericNumber.zeroValue = 0;
myGenericNumber.add = function (x, y) { return x + y };

This makes it easy to create flexible classes that adapt to multiple data types.

🔒 Generic Constraints

Sometimes, you want to restrict what kinds of types can be passed to a generic.
You can use constraints with the extends keyword. 

interface Lengthwise {
  length: number;
}

function loggingIdentity<T extends Lengthwise>(arg: T): T {
  console.log(arg.length); // Safe access to 'length'
  return arg;
}

Now, only types that have a length property (like strings or arrays) can be used with this function.

🌐 Generics in TypeScript APIs

When building APIs, generics make your functions and interfaces reusable and type-safe — especially for API responses that share the same structure but contain different data types.

1. Generic Interface for API Response

interface ApiResponse<T> {
  success: boolean;
  data: T;
  message?: string;
}

Here, T represents the type of data returned by the API — it can be User[], Product[], or anything else.

2. Generic Function for Fetching Data

async function fetchData<T>(url: string): Promise<ApiResponse<T>> {
  const response = await fetch(url);
  if (!response.ok) {
    throw new Error(`HTTP error! status: ${response.status}`);
  }
  const result: ApiResponse<T> = await response.json();
  return result;
}

This function adapts to any API endpoint while maintaining strict type safety.

3. Using Generic Types with Different Data

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

interface Product {
  id: number;
  name: string;
  price: number;
}

// Fetch Users
async function getUsers() {
  try {
    const userResponse = await fetchData<User[]>('/api/users');
    console.log(userResponse.data); // Type: User[]
  } catch (error) {
    console.error(error);
  }
}

// Fetch Products
async function getProducts() {
  try {
    const productResponse = await fetchData<Product[]>('/api/products');
    console.log(productResponse.data); // Type: Product[]
  } catch (error) {
    console.error(error);
  }
}

💡 Benefits of Using Generics in APIs

✅ Type Safety: Prevents type mismatches and runtime errors.
✅ Reusability: Use the same logic for different endpoints.
✅ Reduced Code Duplication: Write once, use everywhere.
✅ Improved Readability: Clearly define the shape and type of expected data.

🚀 Conclusion

Generics are one of the most powerful features in TypeScript.
They make your code cleaner, safer, and more scalable — perfect for building robust applications or APIs.

Whether you’re creating utility functions, classes, or API handlers, mastering generics will level up your TypeScript skills and help you write professional, production-ready code.

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