To get the “available” area of the screen (i.e. not including any bars on the edges of the screen, but including window chrome and other windows:

var availableArea = {
    pos: {
        x: window.screen.availLeft,
        y: window.screen.availTop
    },
    size: {
        width: window.screen.availWidth,
        height: window.screen.availHeight
    }
};