GriffLab GriffLab - 6 months ago 48
Javascript Question

How can I get the mouse coordinates relative to a parent div? Javascript

I currently have a div structured with other elements inside of it.

Something similar to below;

<div id="container" style="position: relative; width: 500px; height: 500px;">
<div style="position: absolute; left: 50px; top: 50px;"></div>
<div style="position: absolute; left: 100px; top: 100px;"></div>

I am trying to get the mouse position relative to the div with the id

So far I have this;

function onMousemove(event) {

x = event.offsetX;
y = event.offsetY;

var elem = document.getElementById("container");
elem.addEventListener("mousemove", onMousemove, false);

This works fine if the div with the id
has no children. When the
div has children it gets the mouse co-ordinates relative to the child rather than the parent.

What I mean by this is if the mouse was at a position of
x: 51, y: 51
relative to the parent div, it would actually return
x: 1, y: 1
relative to the child div, using the html given above.

How can I achieve what I want, no libraries please.


tymeJV has kindly made a jsfiddle of what is happening above.

Answer Source

In essence: 'mouseposition' - 'parent element position' = 'mouseposition relative to parent element'

So here I modified your function:

function onMousemove(e){
    var m_posx = 0, m_posy = 0, e_posx = 0, e_posy = 0,
           obj = this;
    //get mouse position on document crossbrowser
    if (!e){e = window.event;}
    if (e.pageX || e.pageY){
        m_posx = e.pageX;
        m_posy = e.pageY;
    } else if (e.clientX || e.clientY){
        m_posx = e.clientX + document.body.scrollLeft
                 + document.documentElement.scrollLeft;
        m_posy = e.clientY + document.body.scrollTop
                 + document.documentElement.scrollTop;
    //get parent element position in document
    if (obj.offsetParent){
        do { 
            e_posx += obj.offsetLeft;
            e_posy += obj.offsetTop;
        } while (obj = obj.offsetParent);
    // mouse position minus elm position is mouseposition relative to element:
    dbg.innerHTML = ' X Position: ' + (m_posx-e_posx) 
                  + ' Y Position: ' + (m_posy-e_posy);

var elem = document.getElementById('container');
elem.addEventListener('mousemove', onMousemove, false);

var dbg=document.getElementById('dbg');  //debut output div instead of console

Here is a working demo fiddle. As you can read in the code, this also looks at the document's scroll position.

PPK's article on 'event properties' and 'find position' are (as always) a good read!

Hope this helps!

I actually found an error in ppk's code (how rare is that?!): I corrected the erroneous var in:
if (!e) var e = window.event; to if (!e){e = window.event;}

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