What is the difference between Types and Interfaces in TypeScript?

Explanation & Concept

Both `type` and `interface` define object contracts, but they differ in capabilities:

• Interfaces: Can be declared multiple times to merge declarations (declaration merging). Ideal for public library APIs and standard object contracts. • Type Aliases: More expressive. Can represent union types (`type ID = string | number`), primitives, tuples, and mapped types. Cannot be re-opened for merging.

Code Example

typescript

// Interface declaration merging
interface User {
  id: string;
  name: string;
}
interface User {
  role: "admin" | "member"; // Automatically merged!
}

// Type alias with union & intersection
type Status = "pending" | "approved" | "rejected";
type UserWithStatus = User & { status: Status };

const activeUser: UserWithStatus = {
  id: "u_1",
  name: "Jordan",
  role: "admin",
  status: "approved"
};

How It Works & Key Takeaways

Use `interface` for object models and public API definitions. Use `type` when union types, tuples, or complex conditional transformations are required.

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typescript

What is the difference between Types and Interfaces in TypeScript?

Explanation & Concept

Both `type` and `interface` define object contracts, but they differ in capabilities:

Code Example

typescript

// Interface declaration merging
interface User {
  id: string;
  name: string;
}
interface User {
  role: "admin" | "member"; // Automatically merged!
}

// Type alias with union & intersection
type Status = "pending" | "approved" | "rejected";
type UserWithStatus = User & { status: Status };

const activeUser: UserWithStatus = {
  id: "u_1",
  name: "Jordan",
  role: "admin",
  status: "approved"
};

How It Works & Key Takeaways

Use `interface` for object models and public API definitions. Use `type` when union types, tuples, or complex conditional transformations are required.

Mock Test · 1 Questions · Optional

Ready for Generics?

Take a quick test to evaluate yourself, or move straight to the next module.

Beginner2 questions

Skip