Timer with callback in JavaScript

timer.js

// See e.g: http://www.sitepoint.com/creating-accurate-timers-in-javascript/
//          http://stackoverflow.com/questions/3969475/javascript-pause-settimeout
Timer = function(duration, resolution, delegates) {
    'use strict';

    duration   || (duration   = 1000);
    resolution || (resolution = 20);
    delegates  || (delegates  = {});

    delegates.onStart    || (delegates.onStart    = function(){} );
    delegates.onProgress || (delegates.onProgress = function(){} );
    delegates.onStop     || (delegates.onStop     = function(){} );

    var steps = Math.max(1, (duration / 100) * (resolution / 10)),
        speed = duration / steps,
        count,
        start,
        timeoutId;


    this.run = function() {
        killTimer()
        count = 0;
        start = new Date().getTime();
        delegates.onStart(steps, count);
        doTimer();
    };    

    this.resume = function() {
        start = new Date().getTime();
        doTimer();
    };  

    this.disconnect = function() {
        killTimer()
    };  

    function killTimer() {
        if(timeoutId) {
            window.clearTimeout(timeoutId);
            timeoutId = undefined;
        }
    };

    function doTimer() {
        if(++count == steps) {
            delegates.onStop(steps, count);
        }
        else {
            var pause = delegates.onProgress(steps, count);
            if(!pause) {
                var diff = (new Date().getTime() - start);

                if(diff > duration) {
                    delegates.onStop(steps, count);
                }
                else {
                    diff -= (count * speed);
                    timeoutId = window.setTimeout(doTimer, (speed - diff));
                }
            }
        }
    };

    return this;
};


// Run the timer and compare to timer without adjustment
(function(){
    var d,
        n,
        stepTimeout = function() {
            console.log("\n* StepTimeout start");
            n = 0;
            d = new Date();
            var j = 0;

            function doSillyWork() {
                // Do some silly work
                for (var c = 0; c < Math.floor((Math.random() * 1000000) + 100000); c++) {
                    n++
                }

                if(!(j % 6)) {
                    console.log("  StepTimeout progress. steps:" + j);
                }

                if(++j == 50) {
                    // Should have duration >>> Timer duration 
                    console.log("* StepTimeout finished @ " + (new Date() - d) + "ms. n: " + n);
                }
                else {
                    window.setTimeout(doSillyWork, 40);
                }
            };

            window.setTimeout(doSillyWork, 40);
        },
        oneTimeout = function() {
            console.log("\n* Single timeout start");
            d = new Date();
            window.setTimeout(function() {
                // No workload. Should have  duration ~ Timer duration
                console.log("* Single timeout finished @ " + (new Date() - d) + "ms"); 
                stepTimeout();
            }, 2000);
        };


    new Timer(2000, 30, {
        onStart: function(steps, count) { 
            n = 0;
            d = new Date();
            console.log("* Timer start");
            console.log("  Timer progress. steps:" + steps + ", count: " + count);
        }, 
        onProgress: function(steps, count) {
            if(!(count % 6)) {
                console.log("  Timer progress. steps:" + steps + ", count: " + count);
            }

            // Do some silly work
            for (var i = 0; i < Math.floor((Math.random() * 1000000) + 100000); i++) {
                n++
            }
        },
        onStop: function(steps, count) { 
            console.log("  Timer progress. steps:" + steps + ", count: " + count);
            if(count < steps) {
                console.log("  * Timer finished before work was done!");
            }
            console.log("* Timer finished @ " + (new Date() - d) + "ms. n: " + n);
            oneTimeout();
        } 
    }).run();
})();