用JavaScript将给定值除以.style.width

我需要在Javascript中划分.style.width函数给定的值。 我的代码如下所示:

    var x = document.getElementsByClassName("item");
    var i;

    var v = x[0].style.width; 

In variable 'v' I have some value in pixels, e.g. 200px width. I would need to divide this value by 2 to get 100px. But this formula doesn't work: var v = x[0].style.width / 2;

我究竟做错了什么?感谢帮助。