Interface vs Type Alias in TypeScript

For a plain object shape, interface and type are usually interchangeable. interface User { name: string } and type User = { name: string } express the same structural requirement. Neither declaration creates a runtime value or validates data at runtime.

A type alias can name any type, including a union, primitive, tuple, or object type. An interface names an object type. Both can compose object shapes: an interface commonly uses extends, while a type alias can use an intersection such as A & B. An interface cannot extend a type with a conflicting property. Same-name interface declarations also fail when non-function properties conflict. By contrast, an intersection may compile while reducing an incompatible property to never.

Interfaces are open to declaration merging. Separate declarations with the same name combine when their members are compatible, while redeclaring a type alias is an error. That makes an interface a natural choice when consumers are meant to augment a public shape. Use a type alias for unions and other types an interface cannot name. For an ordinary object shape that does not need merging, either is reasonable, so following the codebase convention is usually the clearest choice.