TS2339: Property does not exist on type

Add the property to the type or interface definition.

interface User {
  id: number
  name: string
}

// ❌ Property email does not exist on type User
console.log(user.email)

// ✅ Add it to the interface
interface User {
  id: number
  name: string
  email: string  // Added
}

Mark properties as optional with ? when they may not always be present.

interface Config {
  apiUrl: string
  timeout?: number  // Optional
  retries?: number  // Optional
}

const config: Config = { apiUrl: "https://api.example.com" } // ✅ No timeout required

// Access with optional chaining
const timeout = config.timeout ?? 5000  // Default if undefined

Use an index signature when objects have dynamic string keys.

// For objects with dynamic keys
interface Dictionary {
  [key: string]: string | number
}

const dict: Dictionary = {}
dict["anyKey"] = "anyValue"  // ✅ Allowed

// Or use Record utility type
const dict: Record<string, string | number> = {}
dict["key"] = 42  // ✅

Use 'as' to tell TypeScript the type when you know more than the compiler.

// When you know the property exists at runtime
const data: unknown = JSON.parse(response)

// ❌ Property name does not exist on unknown
console.log(data.name)

// ✅ Type assertion
const user = data as { name: string; email: string }
console.log(user.name)