var canvas = document.getElementById('mundo');

​

function getMousePos(canvas, evt){

    var rect = canvas.getBoundingClientRect();

    return{

        x: evt.clientX - rect.left,

        y: evt.clientY - rect.top

    };

}

​

canvas.addEventListener('mousemove', function(evt){

    var mousePos = getMousePos(canvas, evt);

    msj = ('Mouse position: ' + mousePos.x + ', ' + mousePos.y);

    document.getElementById('msj').innerHTML = msj;

}, false);