text: class User {
      constructor(firstName = undefined, lastName = undefined,
          email = undefined, password = undefined) {
          this.firstName = firstName;
          this.lastName = lastName;
          this.email = email;
          this.password = password;
      }
  
      fromJson(obj) {
          this.firstName = obj.firstName;
          this.lastName = obj.lastName;
          this.email = obj.email;
          this.password = obj.password;
          return this;
      }
  
      toJson() {
          return {
              'firstName': this.firstName,
              'lastName': this.lastName,
              'email': this.email,
              'password': this.password
          };
      }
  }
  