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Interfaces & Type Aliases
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Interfaces & Type Aliases

Define the shape of objects with interfaces and type aliases.
10 min read+150 XP on completionCert: TypeScript Crash Course
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Interfaces & Type Aliases

TypeScript interfaces and type aliases define the shape of objects — the contract that code must satisfy.

Interface

interface User {
  id: string
  name: string
  email: string
  role: 'admin' | 'student'
  bio?: string         // optional
}

const user: User = {
  id: 'user_001',
  name: 'Jordan Morris',
  email: 'jordan@jsupremetech.com',
  role: 'admin',
}

Type Alias

type User = {
  id: string
  name: string
}

// Type aliases can do things interfaces cannot
type StringOrNumber = string | number
type Callback = (err: Error | null, result: string) => void

Extending Interfaces

interface Entity {
  id: string
  createdAt: Date
}

interface Course extends Entity {
  title: string
  xp: number
}

readonly Properties

interface Config {
  readonly apiUrl: string   // cannot change after creation
  debug: boolean            // can change
}

const config: Config = { apiUrl: '/api', debug: false }
// config.apiUrl = '/other'  // Error: read-only
config.debug = true          // OK

Intersection Types

type AdminUser = User & { permissions: string[] }
type BaseEntity = { id: string } & { createdAt: Date }
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