JSFiddle - React, Tailwind, and code Playground

JavaScript

var parents = [
  { 
    id: 77777,
    data: {}
  },
  { 
    id: 88888,
    data: {}
  },
  { 
    id: 99999,
    data: {}
  }
]


var children = [
  {
    belongTo: 77777,
    data: [{
      id: 111,
      data: {}
    },
    {
      id: 222,
      data: {}
    }]
  },
  {
    belongTo: 99999,
    data: [{
      id: 333,
      data: {}
    }]
  }
]
var all = [];
parents.forEach(function(e, k) {
	all.push(e);
    children.forEach(function(el, key) {        
        if(el.belongTo == e.id) {
            el.data.forEach(function(element) {
                all.push(element);            
            });
            
        }
    });
});
console.log(all);