TypeScript Interfaces

Overview

Interfaces describe the shape of objects. They support optional and readonly properties, method signatures, and extension. Use interfaces for public APIs and object-oriented patterns; type aliases overlap for many use cases.

Syntax / Usage

interface User {
  id: string
  name: string
  email?: string
  readonly createdAt: Date
}

interface Admin extends User {
  permissions: string[]
}

function greet(user: User): string {
  return `Hello, ${user.name}`
}

// Index signature for dynamic keys
interface StringMap {
  [key: string]: string
}

// Implementing in a class
class DbUser implements User {
  id: string
  name: string
  createdAt: Date
  constructor(id: string, name: string) {
    this.id = id
    this.name = name
    this.createdAt = new Date()
  }
}

Examples

API response contract:

interface PaginatedResponse<T> {
  data: T[]
  page: number
  total: number
}

interface Post {
  id: string
  title: string
}

const res: PaginatedResponse<Post> = await fetchPosts()

Function type in an interface:

interface Logger {
  log(message: string): void
  error(message: string, code?: number): void
}

Common Mistakes

• Declaring optional fields without handling undefined at use sites
• Using interfaces for unions—use type instead
• Duplicate interface merging surprises in global augmentation
• Extending interfaces when composition (&) is clearer

See Also

basic-types generics utility-types unions