answer to https://gist.github.com/LucasRoesler/3023c58dd64fd2b14f31f25f5afd543a

gistfile1.txt

const collection = [
  { id: 1, name: “Han Solo”, title: “Scruffy Looking Nerf Herder”, allegiance: “rebellion” },
  { id: 2, name: “Darth Vader”, title: “Sith Lord”, allegiance: “empire” },
  { id: 3, name: “K-2SO”, title: “Sass-bot”, allegiance: “rebellion” },
  { id: 4, name: “Wilhuff Tarkin”, title: “Grand Moff”, allegiance: “empire” },
];

var sortedCollection = collection.sort(function(a,b){
	return a.name.localeCompare(b.name);
});

var idsOnly=collection.map(function(item){
	return item.id;
});

var mutatedCollection=collection.map(function(item){
	item.name=item.name+" - deceased";
  return item;
});

collection.shift();

function groupBy( array , f )
{
  var groups = {};
  array.forEach( function( o )
  {
    var group = JSON.stringify( f(o) );
    groups[group] = groups[group] || [];
    groups[group].push( o );  
  });
  return Object.keys(groups).map( function( group )
  {
    return groups[group]; 
  })
}
groupBy(collection,"allegiance")