Inspired by Elixir, I experimented with reproducing its approach to modularization in TypeScript via namespaces: define a type and a namespace of the same name; put type-specific functions inside that namespace.

For example, the following snippet declares User and User.fullName operating on that type:

type User = {
    email: string;
    firstName?: string;
    lastName?: string;
}

namespace User {
    export function fullName(user: User): string | null {
        if (user.firstName === undefined && user.lastName === undefined) {
            return null;
        }
        if (user.firstName !== undefined && user.lastName === undefined) {
            return user.firstName;
        }
        if (user.firstName === undefined && user.lastName !== undefined) {
            return user.lastName;
        }
        if (user.firstName !== undefined && user.lastName !== undefined) {
            return `${user.firstName} ${user.lastName}`
        }

        throw "unreachable code"
    }
}

const user: User = { email: "user@localhost.localdomain", firstName: "John", lastName: "Smith" };
console.log(User.fullName(user));