Data Transfer Object for the User type

UserDto.ts

export class UserDto {
  static from(input: unknown) {
    const parsedInput = typeof input === 'string' ? JSON.parse(input) : input;
    
    if(!UserDto.isValid(parsedInput)) {
      throw Error('Invalid input type');
    }
    
    // at this point `parsedInput` is guaranteed to match `UserDto`
    const { firstName, lastName, isFormal } = parsedInput;

    return new UserDto(firstName, lastName, isFormal);
  }

  // note the use of `is` to type guard to `UserDto`
  protected static isValid(input: any) : input is UserDto {
    return (
      typeof input.firstName === 'string' &&
      typeof input.lastName === 'string' &&
      typeof input.isFormal === 'boolean'
    );
  }

  constructor(
    readonly firstName: string,
    readonly lastName: string,
    readonly isFormal: boolean
  ) { }
  
  serialize() {
    return JSON.stringify(this);
  }
}