Today, there is a problem in the mobile event of writing a custom scroll bar with native JS. The scene is to click a div and move the div (ID: scroll_box) to make the scroll bar of the div beyond the fixed height move with it. The general normal logic is as follows:

let oScrollBox = document.getElementById('scroll_box');

oScrollBox.onmousedown = function(e) {
　　let event1 = e || event;
　　let positionY = event1.clientY - this.offsetTop;
　　document.onmousemove = function(ev) {
　　　　let event2 = ev || event;
　　　　var divY = event2.clientY - positionY;
　　　　oScrollBox.style.top = divY + 'px';

　　}

　　document.onmouseup = function() {
　　　　document.onmousemove = null;
　　};
}

At this time, the error will be reported.  

The event2 = ev || event; in the document.onmousemove method causes the error here, because the event variable is already used in the let event1 = e || event; above, so an error is reported. The solution is to write document.onmousemove into a separate In the method, so that you can avoid the variable duplication problem, the code is as follows:

oScrollBox.onmousedown = function(e) {
　　let event1 = e || event;
　　let positionY = event1.clientY - this.offsetTop;
　　document.onmousemove = function(ev) {
　　　　mouseMove(oScrollBox,ev,positionY);
　　}
　　document.onmouseup = function() {
　　　　document.onmousemove = null;
　　}
};
function mouseMove(obj,e,positionY) {
　　let event2 = e || event;
　　var divY = event2.clientY - positionY;
　　obj.style.top = divY + 'px';
}

This can be solved perfectly!