How to extend a class (add a new attribute to the class object) programmatically?

Question

JavaScript

The following situation:
Assume there are the following classes (TypeScript pseudocode):

class Project {
  id: number;
  name: string;
  stages: Stage[];
  constructor() {
    this.stages = []
  }
}

class Stage {
  id: number;
  name: string;
  jobs: Job[];
  constructor() {
    this.jobs= []
  }
}

class Job {
  id: number;
  name: string;
}

We have a list of projects (Project[]), to which work stages are attached (Stage[]), to which specific jobs are attached (Job[]).
There is a form with a list of projects, you can fall into a specific project. On the project change form, you can bind (add) specific stages and works to the project by marking them with checkboxes. But the Stage and Jobs classes do not have the checked attribute: boolean. How to programmatically add an attribute? How and where to store objects that have checked === true, so that later with a put request to update the project on the backend?
Here is a small example of a component in Angular:

@Component 
export class ProjectFormComponent implements OnInit {
 constructor (private projectService: ProjectService) { }
 projects: Projects[];
 
 onInit() {
   this.projectService.getPorjects()
             .subscribe(projects => this.projects = projects);
 }

 OnEditProjectButtonClick(project) {
    this.projectService.getStageByProjectID(project)
               .subscribe(stages => {
                  project.stages = stages (как то нужно добавить атрибут checked к объекту stages)
                  project.stages.filter(stage => stage.checked) (но у нас нет свойства checked)
             }
  }
}

Sample:

<ul>
    <li *ngFor="let stage of project.stages">
       <input type="checkbox" [checked] = "stage.checked" (вот здесь необходимо свойство checked, а его нет в классе)
        {{stage.name}}
    </li>
</ul>

Answer 1

can be used. Or Finally, you can define the IChecked interface and type it:stage['checked']
stage: Stage & IChecked