var btn = document.getElementById('btn');
var div1 = document.getElementById('div1');
var tiemr;
btn.onclick = function(){
tiemr = setInterval(function(){
var speed = (300 - div1.offsetLeft)/10;
// speed = Math.ceil(speed); //向上取整
speed = speed>0?Math.ceil(speed):Math.floor(speed);//大于零向上，小于零向下
if (div1.offsetLeft == 300) {
clearInterval(tiemr);
} else {
div1.style.left = div1.offsetLeft + speed + 'px';
}
},30)
}
