JSFiddle - React, Tailwind, and code Playground

HTML

{<span id='output'></span>}

JavaScript

function Node(id, children){
  this.id = id;
  this.children = children || [];
  this.countParents = function(nodeList){
    var count = 0;
    for(var i=0;i<nodeList.length;i++){
      if(nodeList.children.indexOf(this.id) > -1)
        count++;
    }
    return count;
  };
};
function NodeList(nodes){
  this.list = nodes || [];
  this.parentCounts = function(){
    var output = [];
    for(var i=0;i<this.list.length;i++){
      var count = 0;
      for(var e=0;e<this.list.length;e++){
        if(this.list[e].children.indexOf(this.list[i].id) > -1)
          count++;
      }
      output.push(this.list[i].id+":"+count);
    }
    return output.join(", ");
  }
};

var nodes = new NodeList([
  new Node(11, [12]),
  new Node(12, [13,14]),
  new Node(13, [15]),
  new Node(14, [15]),
  new Node(15, [16,17,18]),
  new Node(16),
  new Node(17),
  new Node(18, [19]),
  new Node(19)
]);

document.getElementById("output").innerHTML = nodes.parentCounts();