how to get mouse coordinates in javascript

<p>Click in the div element below to get the x (horizontal) and y (vertical) coordinates of the mouse pointer, when it is clicked.</p>

<div onclick="showCoords(event)"><p id="demo"></p></div>

<p><strong>Tip:</strong> Try to click different places in the div.</p>

<script>
function showCoords(event) {
  var cX = event.clientX;
  var sX = event.screenX;
  var cY = event.clientY;
  var sY = event.screenY;
  var coords1 = "client - X: " + cX + ", Y coords: " + cY;
  var coords2 = "screen - X: " + sX + ", Y coords: " + sY;
  document.getElementById("demo").innerHTML = coords1 + "<br>" + coords2;
}
</script>// Cursor coordinate functions
var myX, myY, xyOn, myMouseX, myMouseY;
xyOn = true;

function getXYPosition(e) {
    myMouseX = (e || event).clientX;
    myMouseY = (e || event).clientY;
    if (document.documentElement.scrollTop > 0) {
        myMouseY = myMouseY + document.documentElement.scrollTop;
    }
    if (xyOn) {
        alert("X is " + myMouseX + "\nY is " + myMouseY);
    }
}
function toggleXY() {
    xyOn = !xyOn;
    document.getElementById('xyLink').blur();
    return false;
}   

document.onmouseup = getXYPosition;