Collapsible accordion in angular

Keep your function in ngAfterViewInit instead of ngOnInit. See updated stackblitz

The problem is that on ngOnInit the view is not completely painted, and you do not get all the elements on which you want to bind the function.

ngAfterViewInit() {
    var acc = document.getElementsByClassName("accordion");
    var i;

    for (i = 0; i < acc.length; i++) {
      acc[i].addEventListener("click", function () {
        this.classList.toggle("active");
        var panel = this.nextElementSibling;
        if (panel.style.maxHeight) {
          panel.style.maxHeight = null;
        } else {
          panel.style.maxHeight = panel.scrollHeight + "px";
        }
      });
    }
  }

Using angular do it like shown below.

Keep a click function on the button and bind a property isActive to the corresponding array element. Then show/hide the accordian based on if isActive has value true/false.

<div *ngFor="let item of data;let i = index;">
  <button class="accordion" (click)="toggleAccordian($event, i)"> {{item.parentName}} </button>
  <div class="panel" *ngFor="let child of item.childProperties" hide="!item.isActive">
  <p> {{child.propertyName}} </p>
</div>
</div>


toggleAccordian(event, index) {
      var element = event.target;
      element.classList.toggle("active");
      if(this.data[index].isActive) {
        this.data[index].isActive = false;
      } else {
        this.data[index].isActive = true;
      }      
      var panel = element.nextElementSibling;
      if (panel.style.maxHeight) {
        panel.style.maxHeight = null;
      } else {
        panel.style.maxHeight = panel.scrollHeight + "px";
      }
  }