TypeScript's type system is one of its greatest strengths, enabling developers to write robust, maintainable, and scalable code. Among the advanced features that TypeScript offers, Generics stand out as a powerful tool for creating reusable, flexible, and type-safe components. Generics allow you to write code that can work with different types while still maintaining strong typing.

In this blog post, we’ll explore what Generics are, why they are important, and how you can use them to level up your TypeScript development.

What Are Generics?

Generics are a way to write functions, interfaces, or classes that can work with multiple types rather than being restricted to a single one. They allow you to define placeholders (or type variables) that can be replaced with specific types later, offering flexibility without sacrificing type safety.

Generics are like the "variables" of types: you define a generic type when writing the function or class, and then specify the actual type when you use it.

Why Use Generics?

Generics are particularly useful when:

• You want to create reusable components.

• You need type safety with a variety of data types.

• You want your code to be as flexible as possible without sacrificing strict typing.

Generics allow you to maintain both flexibility and safety, ensuring that even with varying types, TypeScript will catch any potential type-related issues.

Basic Generic Syntax

Generics in TypeScript are defined using angle brackets <> with a placeholder name (commonly T for "Type").

Here’s a simple example to illustrate the concept of a generic function:

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

// Using the generic function with different types
const num = identity<number>(42); // num is of type number
const str = identity<string>("Hello!"); // str is of type string

In this example, the function identity uses a generic type T. When the function is called, TypeScript knows that T will be replaced by the actual type passed in (number or string in this case).

Using Generics in Functions

Generics allow functions to operate on a variety of types. Let’s see how we can use generics to write a function that works for arrays of different types:

Example:

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

// Use the generic function with different types
const firstNumber = getFirstElement([1, 2, 3]); // Inferred type: number
const firstString = getFirstElement(["apple", "banana", "cherry"]); // Inferred type: string

Here, the generic type T ensures that the function can handle an array of any type (T[]), and TypeScript infers the correct return type based on the input.

Generics in Interfaces

Generics also work well with interfaces, providing additional flexibility when creating reusable, type-safe data structures.

Example:

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interface Box<T> {
  contents: T;
}

const numberBox: Box<number> = { contents: 100 };
const stringBox: Box<string> = { contents: "Hello" };

console.log(numberBox.contents); // 100
console.log(stringBox.contents); // Hello

In this case, the Box interface is generic, meaning that the contents property can be of any type. This allows us to create Box objects that store numbers, strings, or any other type.

Generics in Classes

Generics can also be used to make classes more flexible. By introducing a generic parameter, you can create reusable class components that work with different types.

Example:

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class DataStorage<T> {
  private data: T[] = [];

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

  removeItem(item: T): void {
    this.data = this.data.filter((element) => element !== item);
  }

  getItems(): T[] {
    return [...this.data];
  }
}

// Using the generic class
const textStorage = new DataStorage<string>();
textStorage.addItem("Hello");
textStorage.addItem("World");
console.log(textStorage.getItems()); // ['Hello', 'World']

const numberStorage = new DataStorage<number>();
numberStorage.addItem(10);
numberStorage.addItem(20);
console.log(numberStorage.getItems()); // [10, 20]

In this example, DataStorage is a generic class where the type T can be specified when the class is instantiated. This allows you to store data of different types, and TypeScript will enforce the correct types during usage.

Constraints in Generics

Sometimes, you may want to impose constraints on your generic types to ensure that they meet certain criteria. For instance, you may want to enforce that a type has certain properties or methods.

Example: Using extends to Add Constraints

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interface Lengthwise {
  length: number;
}

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

// Valid: item has a length property
logLength("Hello!"); // 6
logLength([1, 2, 3]); // 3

// Invalid: number doesn't have a length property
// logLength(10); // Error

In this example, the generic type T is constrained to types that have a length property by using T extends Lengthwise. This prevents invalid types (such as number) from being used.

Default Types for Generics

You can also provide default types for generics, which will be used if no specific type is provided.

Example:

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function createPair<T = string, U = number>(first: T, second: U): [T, U] {
  return [first, second];
}

const pair1 = createPair("hello", 42); // [string, number]
const pair2 = createPair(100, 200); // [number, number]
const pair3 = createPair("default"); // [string, number] with default U type

Here, T defaults to string and U defaults to number if no type is provided. This allows you to create pairs with varying types while maintaining default behavior.

Generics with Multiple Type Parameters

Generics also allow for multiple type parameters when working with more complex relationships between types.

Example:

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function mergeObjects<T, U>(obj1: T, obj2: U): T & U {
  return { ...obj1, ...obj2 };
}

const person = mergeObjects({ name: "Alice" }, { age: 30 });
console.log(person); // { name: 'Alice', age: 30 }

In this example, mergeObjects takes two objects and combines them into one, with the resulting object having both properties from T and U. TypeScript ensures that the return value includes the properties from both types.

Why Generics Matter

Generics in TypeScript offer several key benefits:

1. Code Reusability: With generics, you can create flexible, reusable components that work with different data types without duplicating code.

2. Type Safety: Generics maintain type safety, allowing TypeScript to catch type errors during compile-time even with flexible code.

3. Self-Documenting: Code that uses generics is often more readable and self-documenting, as it clearly indicates that the function or class can handle various types.

Conclusion

Generics are an incredibly powerful feature in TypeScript that help make your code flexible, reusable, and type-safe. Whether you're working with functions, interfaces, classes, or complex data structures, generics allow you to write cleaner, more maintainable code while still enjoying the benefits of TypeScript's type system.

By mastering generics, you can take your TypeScript skills to the next level, enabling you to write versatile components that scale and adapt to various use cases.