var oDiv  = document.getElementsByTagName('div')[0];
        var oSpan = document.getElementsByTagName('span')[0];
        var oBtn  = document.getElementsByTagName('button')[0];
        var timer = null;
        oBtn.onclick = function () {
            clearInterval(timer);
            var iSpeed = oSpan.offsetLeft - oDiv.offsetLeft > 0 ? 7 : -7;
            timer = setInterval(function () {
                if (Math.abs(oSpan.offsetLeft - oDiv.offsetLeft) < Math.abs(iSpeed)) {
                    clearInterval(timer);
                    oDiv.offsetLeft = oSpan.offsetLeft;
                } else {
                    oDiv.style.left = oDiv.offsetLeft + iSpeed + 'px';
                }
            }, 30);
        }