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Working with Interfaces and Generics in TypeScript

bookDesigning Clean and Scalable Types

When designing types in TypeScript, following a set of clear best practices will help you write code that is robust, maintainable, and easy to scale. You should pay special attention to naming conventions, keeping your types DRY ("Don't Repeat Yourself"), and avoiding unnecessary complexity. Naming interfaces and generics clearly is essential: interface names should be descriptive and often start with a capital letter, while generic type parameters are usually single uppercase letters like T, U, or V, but can be more descriptive if the context benefits from it. Strive to make your types reusable and expressive, and always look for opportunities to refactor repetitive patterns into a single, flexible abstraction.


              
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// Define example types
type User = { id: number; name: string };
type Product = { id: number; title: string };

// ❌ Before: repetitive interfaces for paginated results
interface UserPage {
  items: User[];
  total: number;
}

interface ProductPage {
  items: Product[];
  total: number;
}

// ✅ After: reusable generic interface
interface PageResult<T> {
  items: T[];
  total: number;
}

// Examples
const users: PageResult<User> = {
  items: [{ id: 1, name: "Alice" }],
  total: 1
};

const products: PageResult<Product> = {
  items: [{ id: 101, title: "Book" }],
  total: 1
};

console.log(users, products);
copy

Choosing Between Interfaces and Type Aliases

Selecting the right construct—interface or type alias—is crucial for clarity and scalability in your TypeScript code:

  • Use interfaces to describe the shape of objects;
  • Prefer interfaces when you plan to extend or implement types elsewhere;
  • Choose type aliases for primitives, unions, intersections, or tuples;
  • Apply type aliases for more complex or specific type expressions;
  • In large projects, favor interfaces for public APIs and extensible object structures;
  • Use type aliases when combining multiple types or representing non-object types.

Making deliberate choices between interfaces and type aliases ensures your code remains clear, flexible, and easy to maintain as your project evolves.

Tips for Documenting Types

  • Add a descriptive comment above each type definition to clarify its intent;
  • Explain the purpose of each property within interfaces or type aliases using inline comments;
  • Document generic parameters in interfaces to specify what type of data they represent;
  • Note any constraints or special usage instructions to prevent misuse;
  • Keep comments concise and relevant to support quick understanding by other developers.

Good documentation makes your types easier to use, reduces onboarding time for new contributors, and helps you maintain consistency as your project evolves.

1. When should you prefer an interface over a type alias?

2. What's the main benefit of using generics like PageResult<T>?

question mark

When should you prefer an interface over a type alias?

Select the correct answer

question mark

What's the main benefit of using generics like PageResult<T>?

Select the correct answer

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Hvordan kan vi forbedre det?

Tak for dine kommentarer!

Sektion 3. Kapitel 3

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bookDesigning Clean and Scalable Types

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When designing types in TypeScript, following a set of clear best practices will help you write code that is robust, maintainable, and easy to scale. You should pay special attention to naming conventions, keeping your types DRY ("Don't Repeat Yourself"), and avoiding unnecessary complexity. Naming interfaces and generics clearly is essential: interface names should be descriptive and often start with a capital letter, while generic type parameters are usually single uppercase letters like T, U, or V, but can be more descriptive if the context benefits from it. Strive to make your types reusable and expressive, and always look for opportunities to refactor repetitive patterns into a single, flexible abstraction.


              
123456789101112131415161718192021222324252627282930313233

            
// Define example types
type User = { id: number; name: string };
type Product = { id: number; title: string };

// ❌ Before: repetitive interfaces for paginated results
interface UserPage {
  items: User[];
  total: number;
}

interface ProductPage {
  items: Product[];
  total: number;
}

// ✅ After: reusable generic interface
interface PageResult<T> {
  items: T[];
  total: number;
}

// Examples
const users: PageResult<User> = {
  items: [{ id: 1, name: "Alice" }],
  total: 1
};

const products: PageResult<Product> = {
  items: [{ id: 101, title: "Book" }],
  total: 1
};

console.log(users, products);
copy

Choosing Between Interfaces and Type Aliases

Selecting the right construct—interface or type alias—is crucial for clarity and scalability in your TypeScript code:

  • Use interfaces to describe the shape of objects;
  • Prefer interfaces when you plan to extend or implement types elsewhere;
  • Choose type aliases for primitives, unions, intersections, or tuples;
  • Apply type aliases for more complex or specific type expressions;
  • In large projects, favor interfaces for public APIs and extensible object structures;
  • Use type aliases when combining multiple types or representing non-object types.

Making deliberate choices between interfaces and type aliases ensures your code remains clear, flexible, and easy to maintain as your project evolves.

Tips for Documenting Types

  • Add a descriptive comment above each type definition to clarify its intent;
  • Explain the purpose of each property within interfaces or type aliases using inline comments;
  • Document generic parameters in interfaces to specify what type of data they represent;
  • Note any constraints or special usage instructions to prevent misuse;
  • Keep comments concise and relevant to support quick understanding by other developers.

Good documentation makes your types easier to use, reduces onboarding time for new contributors, and helps you maintain consistency as your project evolves.

1. When should you prefer an interface over a type alias?

2. What's the main benefit of using generics like PageResult<T>?

question mark

When should you prefer an interface over a type alias?

Select the correct answer

question mark

What's the main benefit of using generics like PageResult<T>?

Select the correct answer

Var alt klart?

Hvordan kan vi forbedre det?

Tak for dine kommentarer!

Sektion 3. Kapitel 3
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