TypeScript: Interface vs Type

Goal

Use interface until type features are needed. Prefer readable errors and compiler performance.

Rules

• Use interface for object type definitions.
• Use interface extends for extending object shapes.
• Use interface for class contracts.
• Use type for unions, tuples, primitives, mapped types, conditional types, and function aliases.
• Prefer interface extends over object intersections.
• Avoid intersections when properties may conflict.

Examples