window.getComputedStyle()

The Window.getComputedStyle() method returns an object containing the values of all CSS properties of an element, after applying active stylesheets and resolving any basic computation those values may contain.

It’s quite wrong to use ele.style.width to get the element’s width!!!!!! In native JavaScript, you can get a element’s CSS through two ways:

Standard Method

window.getComputedStyle(ele);

For example,

var ele = document.getElementById("content"), // Do not use #
    eleStyle = window.getComputedStyle(ele);
/* Below is the width of ele */
var eleWidth = eleStyle.width;

Why Not Use ele.style?

ele.style is just get the attribule style of ele. If you use ele.style.width, you just get the width of ele.style, not the real width of ele.

If you have done something like:

ele.style.width = "55px"

You get “55px” when using ele.style.width. If you haven’t, you will get undefined.