Angular MutationObserver

mutation-observer.ts

Component code
....
ngAfterViewInit() {
  const config = { attributes: false, childList: true, subtree: true };
  this.messageTreeObserver = new MutationObserver((mutationList, observer) => {
    for (const mutation of mutationList) {
      if (mutation.type === 'childList') {
        // Do action here!
      }
    }
  });

  this.messageTreeObserver.observe(this.chatContent.nativeElement, config);
}

...

ngOnDestroy() {
  if (this.messageTreeObserver) {
    this.messageTreeObserver.disconnect(); //remember to disconnect the obvserver!
  }
}