TS2339: Property 'X' does not exist on type 'Y'

What’s happening?

This error occurs when you try to access a property that doesn’t exist on the type according to TypeScript.

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Reproduction

interface User {
  name: string;
  email: string;
}

const user: User = { name: "Alice", email: "alice@example.com" };
console.log(user.age);
// ❌ Property 'age' does not exist on type 'User'

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Solutions

Option 1: Add the property to the interface

interface User {
  name: string;
  email: string;
  age?: number; // ✅ Made optional
}

const user: User = { name: "Alice", email: "alice@example.com" };
console.log(user.age); // ✅ Returns undefined

Option 2: Use a type assertion (if you know the runtime shape)

interface ExtendedUser extends User {
  age: number;
}

const user = { name: "Alice", email: "alice@example.com", age: 30 };
console.log((user as ExtendedUser).age); // ✅

Option 3: Use index signatures for dynamic properties

interface User {
  name: string;
  email: string;
  [key: string]: any; // ✅ Allows any additional properties
}

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Common scenarios

API responses with extra fields

const response = await fetch('/api/user');
const data = await response.json();
console.log(data.username);
// ❌ Property 'username' does not exist on type 'any'

// ✅ Fix: Define the expected shape
interface ApiResponse {
  username: string;
  email: string;
}

const data: ApiResponse = await response.json();
console.log(data.username); // ✅

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Key takeaways

• Define all properties in your interfaces
• Use optional properties (?) for fields that might not exist
• Prefer type definitions over any

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Additional resources

• Object Types