What is the difference between type and interface

Short answer

interface declares object shapes and supports declaration merging. type creates aliases for any type, including unions and primitives, but cannot be reopened.

Key differences

• Extensibility: Interfaces can be merged by declaring the same name multiple times. Types cannot.
• Unions: type can represent union types (type Status = 'active' | 'inactive'), while interface cannot.
• Implementation: Classes can implement both interface and type aliases that describe object shapes.
• Performance: In general, interfaces produce better error messages and are slightly more performant during type checking in large projects.

When to use each

• Use interface for public API shapes and when you expect declaration merging.
• Use type for unions, tuples, mapped types, and complex type transformations.
• Many teams default to interface for objects and reserve type for utility types.