Javascript Sorting 3 (Sorting messages by date and then group by recipient)

gistfile1.js

var unsorted = [
    {
        recipient: "b",
        timestamp: 1490591976873,
        title: "Hello World 3"
    },
    {
        recipient: "a",        
        timestamp: 1490591722787,
        title: "Hello World"
    },
    {
        recipient: "a",        
        timestamp: 1490591748922,
        title: "Hello World 2"
    },
    {
        recipient: "b",        
        timestamp: 1490591722787,
        title: "Hello World"
    }    
];

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]; 
  })
}

function compare(objA, objB) {
    if (objA.timestamp !== undefined && objB.timestamp !== undefined) {
        if (objA.timestamp < objB.timestamp) {
            return -1;
        } else if (objA.timestamp > objB.timestamp) {
            return 1;
        } else {
            return 0;
        }
    } else {
        // Error
        return;
    }
}

var inbox = [];
var r = unsorted.sort(compare).reverse().reduce(function(reduced, recipient) {
  if (inbox[recipient.recipient] == undefined) {
    inbox[recipient.recipient] = [recipient];
  } else {
    inbox[recipient.recipient].push(recipient);
  }
  console.log(reduced);
  return reduced;
}, {});

console.log(inbox);