JSFiddle - React, Tailwind, and code Playground

JavaScript

var a = "2:50";
var b = "2:15";

var splitTimeStr = function(t){
        var t = t.split(":");
        t[0] = Number(t[0]);
            t[1] = Number(t[1]);
        return t;
};

var addTime = function(t1, t2){
    var t1Hr = splitTimeStr(t1)[0];
    var t1Min = splitTimeStr(t1)[1];
    var t2Hr = splitTimeStr(t2)[0];
    var t2Min = splitTimeStr(t2)[1];
    var rHr = t1Hr + t2Hr;
    var rMin = t1Min + t2Min;
    if (rMin >= 60)
    {
        rMin = rMin - 60;
        rHr = rHr + 1;
    }
    if (rMin < 10) rMin = "0" + rMin;
    if (rHr < 10) rHr = "0" + rHr;
    return "" + rHr + ":" + rMin;
};

document.write(addTime(a, b));