* {
 margin: 0;
 padding: 0;
}
html,
body {
 width: 100%;
 height: 100%;
}
#div1 {
 width: 50px;
 height: 50px;
 background: cyan;
 position: absolute;
}

var div1 = document.querySelector('#div1');
//限制最大寬高，不讓滑塊出去
var maxW = document.body.clientWidth - div1.offsetWidth;
var maxH = document.body.clientHeight - div1.offsetHeight;
//手指觸摸開始，記錄div的初始位置
div1.addEventListener('touchstart', function(e) {
 var ev = e || window.event;
 var touch = ev.targetTouches[0];
 oL = touch.clientX - div1.offsetLeft;
 oT = touch.clientY - div1.offsetTop;
 document.addEventListener("touchmove", defaultEvent, false);
});
//觸摸中的，位置記錄
div1.addEventListener('touchmove', function(e) {
 var ev = e || window.event;
 var touch = ev.targetTouches[0];
 var oLeft = touch.clientX - oL;
 var oTop = touch.clientY - oT;
 if(oLeft < 0) {
 oLeft = 0;
 } else if(oLeft >= maxW) {
 oLeft = maxW;
 }
 if(oTop < 0) {
 oTop = 0;
 } else if(oTop >= maxH) {
 oTop = maxH;
 }
 div1.style.left = oLeft + 'px';
 div1.style.top = oTop + 'px';
});
//觸摸結(jié)束時的處理
div1.addEventListener('touchend', function() {
 document.removeEventListener("touchmove", defaultEvent);
});
//阻止默認事件
function defaultEvent(e) {
 e.preventDefault();
}