准确判断鼠标指针是否在对象里

function mouseMove(ev)
{  
    ev= ev || window.event;
    var mousePos = mouseCoords(ev);
            
    if (currentMenu) {
        var xIn = currentMenu.offsetLeft < mousePos.x && ((currentMenu.offsetLeft + currentMenu.offsetWidth) > mousePos.x);
        var yIn = currentMenu.offsetTop < mousePos.y && ((currentMenu.offsetTop + currentMenu.offsetHeight) > mousePos.y);
                
        if ((xIn && yIn)) {
            document.getElementById('bb').value = 'in';
        } else {
            document.getElementById('bb').value = 'out';
        }
    }
}

function mouseCoords(ev)
{
    if(ev.pageX || ev.pageY){
        return {x:ev.pageX, y:ev.pageY};
    }
    return {
        x:ev.clientX + document.body.scrollLeft - document.body.clientLeft,
        y:ev.clientY + document.body.scrollTop     - document.body.clientTop
    };
}

document.onmousemove = mouseMove;