typescript-interface-vs-type

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

interface User {
  name: string;
}

interface Admin extends User {
  permissions: string[];
}

type Status = "pending" | "approved" | "rejected";
type Point = [number, number];
type Handler = (event: Event) => void;

Why Prefer Extends

• Conflicting properties fail at the definition.
• Error messages are clearer.
• Named interfaces are cached by TypeScript.
• Intersections can be recomputed and harder to debug.

Avoid

type Admin = User & {
  permissions: string[];
};

Use this only when intersection semantics are intentional.

References

• TypeScript Handbook - Everyday Types
• TypeScript Performance Wiki
• Total TypeScript - Intersections vs Interface Extends

Output

• Recommended declaration form.
• Reason: object shape, union, tuple, conditional, mapped type, or extension.
• Any intersection risk.