Interfaces vs Types

Interfaces

Interfaces define the shape of an object. They're the main way to name object types in TypeScript.

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

The ? makes a property optional. Interfaces support declaration merging — you can reopen them to add more properties.

Type Aliases

Type aliases (type keyword) can name any type, including unions, intersections, and primitives. They cannot be reopened.

When to Use Which

Use interfaces when defining object shapes, especially for public APIs and class contracts. Use type aliases for unions, intersections, mapped types, and non-object types.