Angular - Directive - Add a class to an element if the element is contained within an element specified by a selector

is-contained-within.directive.ts

import { Directive, Input, OnChanges, SimpleChanges, ElementRef } from '@angular/core';

@Directive({
  selector: '[isContainedWithin]'
})
export class IsContainedWithinDirective implements OnChanges {

  @Input() isContainedWithin: string = '';
  @Input() classToAdd: string = '';

  constructor(private element: ElementRef) {}

  ngOnChanges(changes: SimpleChanges): void {

    if (!this.element) { return; }

    if (changes.isContainedWithin) {

      const htmlElement = <HTMLElement>this.element.nativeElement;
      const parent = htmlElement.closest(this.isContainedWithin);

      if (this.classToAdd) {
        if (parent) {
          htmlElement.classList.add(this.classToAdd);
        } else {
          htmlElement.classList.remove(this.classToAdd);
        }
      }
    }
  }
}