javascript using reduce function

I have the below array

 ["0,5,p1", "24,29,p2", "78,83,p2", "78,83,p3", "162,167,p2" ] 

i want the output as ["5,p1","10,p2","5,p3"] , so p1..3 are video files paying time with start and end time . 0,5 mean p1 profile played for 5 sec and so on. I want to know what profile take what time in total using ECMA script map,reduce function. Here is what i tried but it doesnt work:

 var ca =   uniqueArray.reduce(function(pval, elem) {
    var spl = elem.split(',');
            var difference = Math.round(spl[1] - spl[0]);
            return difference;
    },elem.split(',')[3]);