javascript - HTML5 canvas screen to isometric coordinate conversion

I am trying to create an isometric game using HTML5 canvas, but don't know how to convert HTML5 canvas screen coordinates to isometric coordinates. My code now is:

var mouseX = 0;
var mouseY = 0;
function mouseCheck(event) {
    mouseX = event.pageX;
    mouseY = event.pageY;
}

which gives me canvas coordinates. But how do I convert these coordinates to isometric coordinates? I am using 16x16 tiles.