What is the difference between type and interface in TypeScript?

In TypeScript, type and interface are often used to describe object types, but there are differences:

• Extensibility:

  • interface can be extended (extends) and merged (declaration merging), meaning multiple declarations with the same interface name are combined.
  • type does not support declaration merging, but you can create unions and intersections of types.

• Capabilities:

  • type is more versatile — it allows describing not only objects but also unions, intersections, primitives, tuples, etc.
  • interface is intended only for describing object structures and classes.

• Syntax and preferences:

  • interface is often preferred for describing public APIs and classes because it supports declarative extension.
  • type is convenient for complex types, such as unions or mapped types.

Example:

interface IUser {
  name: string;
}

interface IUser {
  age: number; // interfaces will merge
}

const user: IUser = { name: 'Alice', age: 30 };

// type won't work this way:
// type User = { name: string };
// type User = { age: number }; // error: Duplicate identifier 'User'

// type allows unions:
type ID = string | number;