Table of Contents

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1. What are Generics?

Generics are a way to create reusable components in TypeScript that work with different types. They allow you to define the shape of a component while deferring the exact type until it is actually used.

This makes your code more flexible and reduces redundancy, allowing functions, classes, or interfaces to work with a variety of types while preserving type safety.

2. How Generics Work in TypeScript

Generics in TypeScript are essentially placeholders for types. Think of them as variables for types, much like how function parameters work. You define a generic parameter and use it in place of the actual type.

Here’s a simple example of a generic function:

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

In this function, T is a generic type parameter. When the function is called, TypeScript will infer the type from the argument passed or you can explicitly pass the type.

// TypeScript infers the type
let output1 = identity(5);    // output1 is of type number
let output2 = identity("hello"); // output2 is of type string

3. Benefits of Using Generics

Here’s why generics are so powerful in TypeScript:

1. Reusability: Write code once, use it for multiple types.
2. Type Safety: Generics help ensure that the correct types are used, reducing the risk of runtime errors.
3. Flexibility: You can create more adaptable components that work across various types.

4. Generic Functions

Let’s expand on the idea of generic functions. You can use generics to make your functions more flexible without sacrificing type safety.

Example: A Generic Array Function

Imagine a function that returns the first element of an array. You want this function to work for any type of array—strings, numbers, objects, etc.

function getFirstElement<T>(arr: T[]): T {
  return arr[0];
}

Here, the generic type T represents the type of the array’s elements, so TypeScript knows exactly what type to return.

let firstString = getFirstElement(["apple", "banana", "cherry"]); // TypeScript knows this is a string
let firstNumber = getFirstElement([10, 20, 30]); // TypeScript knows this is a number

5. Generic Classes

Generics can also be applied to classes, allowing you to build reusable and type-safe classes.

Example: A Generic Stack Class

Here’s a classic example of a stack data structure implemented with generics:

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

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

  pop(): T | undefined {
    return this.items.pop();
  }

  peek(): T | undefined {
    return this.items[this.items.length - 1];
  }
}

Now, you can create stacks for any type:

const numberStack = new Stack<number>();
numberStack.push(10);
numberStack.push(20);
console.log(numberStack.peek()); // Output: 20

const stringStack = new Stack<string>();
stringStack.push("apple");
stringStack.push("banana");
console.log(stringStack.pop()); // Output: "banana"

6. Generic Interfaces

Generics are also useful when defining interfaces. Let’s say you have an interface for a key-value pair, but you want it to work for any type of key and value.

interface KeyValuePair<K, V> {
  key: K;
  value: V;
}

const numberStringPair: KeyValuePair<number, string> = { key: 1, value: "apple" };
const stringBooleanPair: KeyValuePair<string, boolean> = { key: "isActive", value: true };

With generics, the KeyValuePair interface can handle any combination of key-value types, giving you more flexibility in your code.

7. Constraints in Generics

Sometimes, you may want to limit the types that a generic can accept. This is where constraints come in. You can use constraints to specify that a type must extend a particular class or interface.

Example: A Function with a Constraint

Let’s say you want to create a generic function that works with objects that have a length property (e.g., arrays, strings). You can constrain the generic to ensure the type has the length property:

function logLength<T extends { length: number }>(item: T): void {
  console.log(item.length);
}

logLength("Hello");   // Output: 5
logLength([1, 2, 3]); // Output: 3
logLength({ length: 10, value: "example" }); // Output: 10

In this example, the generic type T is constrained to only accept types that have a length property, ensuring type safety.

8. Practical Use Cases of Generics

Generics can be applied in many real-world scenarios to build reusable and safe code. Here are some common use cases:

1. Fetching Data with Type Safety

If you’re fetching data from an API and you want to ensure the correct type is returned:

async function fetchData<T>(url: string): Promise<T> {
  const response = await fetch(url);
  const data = await response.json();
  return data as T;
}

// Using the function
interface User {
  id: number;
  name: string;
}

const user = await fetchData<User>("https://api.example.com/user/1");
console.log(user.name); // TypeScript knows it's a string

2. Reusable Form Validation

You can use generics to create reusable validation logic for forms with different types of fields:

interface FormField<T> {
  value: T;
  validate: () => boolean;
}

const textField: FormField<string> = {
  value: "Hello",
  validate: () => textField.value.length > 0,
};

const numberField: FormField<number> = {
  value: 42,
  validate: () => numberField.value > 0,
};

9. Conclusion

Generics are a fundamental part of TypeScript that allow you to write more reusable, adaptable, and type-safe code. Whether you’re creating functions, classes, or interfaces, using generics ensures that your code works across different types without sacrificing the benefits of static typing.

By leveraging the power of generics, you’ll make your code more maintainable and versatile, enabling you to handle complex scenarios with ease. Try using generics in your next TypeScript project and see how they transform your development experience.

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