Get max value from javascript dictionary object

<script type="text/javascript">

        Array.prototype.getUnique = function () {
            var u = {}, a = [];
            for (var i = 0, l = this.length; i < l; ++i) {
                if (u.hasOwnProperty(this[i])) {
                    continue;
                }
                a.push(this[i]);
                u[this[i]] = 1;
            }
            return a;
        }

        arraysortingxxxx();
        function arraysortingxxxx() {

            var mainarray = [[2, 1920],
  [2, 1080],
  [2, 700],
  [3, 1200],
  [3, 1000],
  [3, 800],
  [4, 1000],
  [4, 900],
  [4, 1920]];


            var arraysingle = new Array();
            for (var i = 0; i < mainarray.length; i++) {
                arraysingle.push(mainarray[i][0])
            }
            var arrayunique = arraysingle.getUnique();
            var arrayfinal = new Array(new Array());
            for (var i = 0; i < arrayunique.length; i++) {
                var array0 = new Array();
                for (var j = 0; j < mainarray.length; j++) {
                    if (mainarray[j][0] == arrayunique[0])
                        array0.push(mainarray[j][1])
                }

                var value = array0.sort(function (a, b) {
                    return a - b;
                });
                arrayfinal.push([arrayunique[i], value[array0.length - 1]]);


            }
            arrayfinal.splice(0, 1);
            var output = arrayfinal;
            debugger;




        };
    </script>