Serializer in TS similar to ActiveModel::Serializer

serializer.ts

abstract class Serializer {
  abstract attributes: string[];

  [customAttribute: string]: unknown; // this allow this[attribute] to work so we can use getters to add extra attributes

  constructor(protected input: Record<string, unknown>) {}

  private filterAttributes() {
    return Object.fromEntries(
      this.attributes.map((attribute) => {
        return [attribute, this.input[attribute] ?? this[attribute]];
      })
    );
  }

  toJSON() {
    return this.filterAttributes();
  }

  toString() {
    return JSON.stringify(this);
  }
}

class UserSerializer extends Serializer {
  attributes = ["firstName", "lastName", "fullName"];

  protected get fullName() {
    return `${this.input.firstName} ${this.input.lastName}`;
  }
}

console.log(
  JSON.stringify(
    new UserSerializer({
      token: "1.2.3",
      firstName: "Sergio",
      lastName: "Xalambrí", // if I remove this, TS should throw, how could I do that?
    })
  )
);

console.log(
  new UserSerializer({
    token: "1.2.3",
    firstName: "Sergio",
    lastName: "Xalambrí",
  }).toString() // calling this is the same as manually calling JSON.stringify
);