HTML canvas resize and mouse coordinates from http://ictknowledge.net/html-canvas-resize-and-mouse-coordinates/

canvasResizeCoordinates.js

canvasName = "mycanvas";

document.getElementById(canvasName).addEventListener('mousemove', function(evt) {
    var xy = getMousePos(evt);

    var position = (xy.x) + ', ' + (xy.y);

    alert(position);
}

function getMousePos(evt) {
    var rect = document.getElementById(canvasName).getBoundingClientRect();

    var X = (evt.clientX - rect.left) / (document.getElementById(canvasName).clientWidth / document.getElementById(canvasName).width);
    var Y = (evt.clientY - rect.top) / (document.getElementById(canvasName).clientHeight / document.getElementById(canvasName).height);
    X = Math.ceil(X);
    Y = Math.ceil(Y);

    return {
        x: X,
        y: Y
    };
}