A functional package model using TypeScript's declaration merging

consumer.ts

// Here I can use `User` as a type, a function, or a namespace
import { User } from "./functional_model";

const sayHi = (user: User): string => (
	`Hi, ${user.name}!`
);

const joe = User({ name: "Joe" });

User.isActive(joe) //=> true
User.State.Deleted //=> "D"
User({}) //=> { name: "", email: "", state: "A" }
sayHi(joe) //=> "Hi, Joe!"

functional_model.ts

export interface User {
	readonly name: string;
	readonly email: string;
	readonly state: User.State;
}

export function User(u: Partial<User>): User {
	return {
		name: "",
		email: "",
		state: User.State.Active,
		...u,
	};
}

export namespace User {
	export const enum State {
		Active              = "A",
		Suspended           = "S",
		Deleted             = "D",
	}
	
	export const isActive = (u: Pick<User, "state">): boolean => (
		u.state === State.Active
	);
}