JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box">

</div>

CSS

#box {
    position: relative;
    width: 200px;
    height: 400px;
    background: red;
    left: 100px;
    top: 50px;
}

JavaScript

document.getElementById("box").onmousedown = function() {
    var posx = 0;
    var posy = 0;
    if (!e) var e = window.event;
    if (e.pageX || e.pageY)     {
        posx = e.pageX;
        posy = e.pageY;
    }
    else if (e.clientX || e.clientY)     {
        posx = e.clientX + document.body.scrollLeft
            + document.documentElement.scrollLeft;
        posy = e.clientY + document.body.scrollTop
            + document.documentElement.scrollTop;
    }
    
    alert("Mouse x: " + posx + "\nMouse y: " + posy);
};