js可拖拽的div

function chatDrag(div1) {
div1.onmousedown = function (ev) {
var oevent = ev || event;
var distanceX = oevent.clientX - div1.offsetLeft;
var distanceY = oevent.clientY - div1.offsetTop;
document.onmousemove = function (ev) {
var oevent = ev || event;
div1.style.left = oevent.clientX - distanceX + 'px';
div1.style.top = oevent.clientY - distanceY + 'px';
};
document.onmouseup = function () {
document.onmousemove = null;
document.onmouseup = null;
};
};
}

var div1 = document.querySelector(".chatContent");
chatDrag(div1);