Microck TypeScript: Interface vs Type
Core Principle
Use interface until you need features from type.
This is the official TypeScript recommendation from the TypeScript Handbook.
When to Use Interface
Use interface for:
- Object type definitions
- Extending other object types
- Class implementations
- Declaration merging (augmenting existing types)
When to Use Type
Use type only when you need:
- Union types:
type Status = 'pending' | 'approved' | 'rejected' - Mapped types:
type Readonly<T> = { readonly [K in keyof T]: T[K] } - Conditional types:
type NonNullable<T> = T extends null | undefined ? never : T - Tuple types:
type Point = [number, number] - Function types (though interface can also work):
type Handler = (event: Event) => void
Prefer interface extends Over Intersection (&)
When extending object types, always prefer interface extends over type intersections.
// Preferred
interface User {
name: string;
}
interface Admin extends User {
permissions: string[];
}
// Avoid
type User = {
name: string;
};
type Admin = User & {
permissions: string[];
};
Reason 1: Better Error Messages
With interface extends, TypeScript raises errors at the definition when extending with incompatible properties:
interface Base {
id: number;
}
// Error immediately at definition
interface Extended extends Base {
id: string; // Error: Type 'string' is not assignable to type 'number'
}
With intersections, errors only appear when accessing the incompatible property, making bugs harder to catch:
type Base = {
id: number;
};
// No error at definition
type Extended = Base & {
id: string;
};
// Error only when used
const item: Extended = { id: 'abc' }; // Error appears here, not at type definition
Reason 2: Better TypeScript Performance
interface extends provides better TypeScript performance:
- Interfaces are cached by name - TypeScript computes the type once and reuses it
- Intersections are recomputed every time they're used, which slows down type checking with complex types
See TypeScript Performance Wiki for details.