var cssProp = {
    'position': 'absolute',
    'top': topPos,
    'left': leftPos
}

//this codes works
cssProp.top += 10;

//this is not..
cssProp.z - index = 10;

cssProp['z-index'] = 10;

var cssProp = {
    'position':'absolute',
    'top': topPos,
    'left': leftPos,
    'zIndex': '0'
};

var cssProp = {'position':'absolute',
                 'top': topPos,
                 'left': leftPos,
                 'z-index': 0
            }