JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = ["2:6", "2:3", "1:1", "1:2", "3:1"];
var tdarray = {};
var newarray = [];

for(var i=0;i<arr.length;i++){
    var data = arr[i].split(":");
    var found = false;
    for(var key in tdarray){
        if(data[0]==key){
            found = true;
            break;
        }
    }
    var list=[];
    if(found){
        list = tdarray[data[0]];
    }
    list.push(data[1]);
    tdarray[data[0]] = list;
}
for(key in tdarray){   
    var data = key + ":" + tdarray[key].join();
    newarray.push(data);
}

console.log(newarray);