groupBy.js

Array.prototype.groupBy = function(field){
  let groupedArr = [];
  this.forEach(function(e){
    //look for an existent group
    let group = groupedArr.find(g => g['field'] === e[field]);
    if (group == undefined){
      //add new group if it doesn't exist
      group = {field: e[field], groupList: []};
      groupedArr.push(group);
    }
    
    //add the element to the group
    group.groupList.push(e);
  });
  
  return groupedArr;
}