What are the differences between `interface` and `type` aliases?

Summary

Both can describe object shapes and be extended. Interfaces support declaration merging and extends, which reads well for object/class contracts; type aliases can name any type, including unions, tuples, primitives, mapped and conditional types.

Longer Answer

Interfaces with the same name in the same scope merge, which is used to augment library types (e.g., adding fields to Window or Express's Request). Type aliases can't be reopened. For object shapes, either is fine; many teams use interface for public object contracts and type for unions and computed types. Interface extends errors on conflicting properties, while intersections (&) may silently produce never properties.

Code Example

interface User { id: string; name: string }
interface User { email: string }        // merged: { id, name, email }

type Status = "active" | "banned";      // only possible with type alias
type WithTimestamps<T> = T & { createdAt: Date; updatedAt: Date };

Key Points

• Interfaces: declaration merging, extends
• Types: unions, tuples, mapped/conditional types
• Both work for object shapes
• Use merging to augment library types