type vs interface: Which Should You Reach For?

TypeScript gives you two ways to name the shape of an object: an interface and a type alias. For a basic object, they look almost identical:

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

type User = {
  name: string;
  age: number;
};

The biggest practical difference is declaration merging. If you declare the same interface twice, TypeScript merges the two into one — a feature libraries use to let you extend existing types. A type alias can't do this: declaring the same name twice is a duplicate identifier error.

interface Box { width: number; }
interface Box { height: number; } // OK — Box now has both

type Cat = { legs: number; };
type Cat = { tail: boolean; }; // Error: Duplicate identifier 'Cat'

The flip side: a type alias can name anything, not just object shapes — unions, primitives, tuples, and mapped types. Only type can write type ID = string | number or type Pair = [number, number]. Interfaces are limited to describing object-like structures. A common rule of thumb: reach for interface when defining the shape of an object or class (especially in a public API where merging or extends helps), and reach for type when you need unions, tuples, or other non-object shapes.