A simple counter for an element containing a number.

jquery.counter.js

/*
Usage:
    <span class="counter">10</span>

$(".counter").countTo({
  step: 5,     // step, integer
  start: 10,
  stop: 20
  length: 500  // total duration in ms
});


*/

(function($){

    var defaultOptions = {
        step: 1,
        length: 1000
    };

    function init ($el, options) {
        var opts = {};
        $.extend(true, opts, defaultOptions, options);
        if (opts.start === undefined) {
            opts.start = parseInt($el.html(), 10) || 0;
        }
        if (opts.stop === undefined) {
            opts.stop = opts.start + opts.step;
        }
        if (opts.start > opts.stop && opts.step > 0) {
            opts.step = -opts.step;
        }
        opts.interval = opts.length / Math.abs(opts.start - opts.stop);
        return opts;
    }

    $.fn.countTo = function(options) {
        var $this = $(this);
        options = init($this, options);
        var counter = options.start;
        var timer = setInterval(function () {
            if ((counter >= options.stop && options.step >= 0) ||
                (counter <= options.stop && options.step <= 0)
               ) {
                clearInterval(timer);
                $this.html(options.stop);
                return;
            }
            counter += options.step;
            $this.html(counter);
        }, options.interval);
    };
})(jQuery);