What is the difference between interface and type in TypeScript? When should each be used?

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

• interface is intended for describing object structures and can be extended (extends) or merged (declaration merging). This is convenient for defining contracts, especially in large codebases or when working with classes.
• type is more versatile: it allows creating aliases for primitives, unions, intersections, tuples, and other types. But type does not support declaration merging.

When to use:

• Use interface if you need to describe an object structure and plan to extend or merge interfaces.
• Use type if you need to describe complex types, unions, intersections, or aliases for primitives.

Example:

interface User {
  name: string;
  age: number;
}

type ID = string | number;

type UserWithID = User & { id: ID };

Thus, interface is better suited for describing objects and extendable contracts, while type is for more complex or composite types.