Interface vs Type Alias

Interface vs Type Alias

Both interface and type can describe object shapes, and for most use-cases they are interchangeable. The choice often comes down to team convention, but there are real differences.

• interface supports declaration merging; type does not
• type can alias unions, intersections, tuples, and primitives; interface cannot
• interface extends with the extends keyword; type composes with &
• Error messages for interface violations are sometimes clearer
• Both compile away to zero runtime cost

// Only possible with type:
type StringOrNumber = string | number;
type Pair = [string, number];

// Only possible with interface:
interface Logger {
  log(msg: string): void;
}
interface Logger {
  warn(msg: string): void; // merged!
}

// Composition comparison:
interface A { a: number; }
interface B extends A { b: string; }

type C = { a: number };
type D = C & { b: string };