Element size and scrolling

There are many JavaScript properties that allow us to read information about element width, height and other geometry features.

We often need them when moving or positioning elements in JavaScript.

Sample element

As a sample element to demonstrate properties we’ll use the one given below:

          <div id="example">
  ...Text...
</div>
<style>
  #example {
    width: 300px;
    height: 200px;
    border: 25px solid #E8C48F;
    padding: 20px;
    overflow: auto;
  }
</style>
        

It has the border, padding and scrolling. The full set of features. There are no margins, as they are not the part of the element itself, and there are no special properties for them.

The element looks like this:

You can open the document in the sandbox.

The picture above demonstrates the most complex case when the element has a scrollbar. Some browsers (not all) reserve the space for it by taking it from the content (labeled as “content width” above).

So, without scrollbar the content width would be 300px, but if the scrollbar is 16px wide (the width may vary between devices and browsers) then only 300 - 16 = 284px remains, and we should take it into account. That’s why examples from this chapter assume that there’s a scrollbar. Without it, some calculations are simpler.

Usually paddings are shown empty on our illustrations, but if there’s a lot of text in the element and it overflows, then browsers show the “overflowing” text at padding-bottom, that’s normal.

Geometry

Here’s the overall picture with geometry properties:

Values of these properties are technically numbers, but these numbers are “of pixels”, so these are pixel measurements.

Let’s start exploring the properties starting from the outside of the element.

offsetParent, offsetLeft/Top

These properties are rarely needed, but still they are the “most outer” geometry properties, so we’ll start with them.

The offsetParent is the nearest ancestor that the browser uses for calculating coordinates during rendering.

That’s the nearest ancestor that is one of the following:

CSS-positioned (position is absolute, relative, fixed or sticky), or
<td>, <th>, or <table>, or
<body>.

Properties offsetLeft/offsetTop provide x/y coordinates relative to offsetParent upper-left corner.

In the example below the inner <div> has <main> as offsetParent and offsetLeft/offsetTop shifts from its upper-left corner (180):

<main style="position: relative" id="main">
  <article>
    <div id="example" style="position: absolute; left: 180px; top: 180px">...</div>
  </article>
</main>
<script>
  alert(example.offsetParent.id); // main
  alert(example.offsetLeft); // 180 (note: a number, not a string "180px")
  alert(example.offsetTop); // 180
</script>

There are several occasions when offsetParent is null:

For not shown elements (display:none or not in the document).
For <body> and <html>.
For elements with position:fixed.

offsetWidth/Height

Now let’s move on to the element itself.

These two properties are the simplest ones. They provide the “outer” width/height of the element. Or, in other words, its full size including borders.

For our sample element:

offsetWidth = 390 – the outer width, can be calculated as inner CSS-width (300px) plus paddings (2 * 20px) and borders (2 * 25px).
offsetHeight = 290 – the outer height.

Geometry properties are calculated only for displayed elements.

If an element (or any of its ancestors) has display:none or is not in the document, then all geometry properties are zero (or null for offsetParent).

For example, offsetParent is null, and offsetWidth, offsetHeight are 0 when we created an element, but haven’t inserted it into the document yet, or it (or its ancestor) has display:none.

We can use this to check if an element is hidden, like this:

          function isHidden(elem) {
  return !elem.offsetWidth && !elem.offsetHeight;
}
        

Please note that such isHidden returns true for elements that are on-screen, but have zero sizes.

clientTop/Left

Inside the element we have the borders.

To measure them, there are properties clientTop and clientLeft.

In our example:

clientLeft = 25 – left border width
clientTop = 25 – top border width

…But to be precise – these properties are not border width/height, but rather relative coordinates of the inner side from the outer side.

What’s the difference?

It becomes visible when the document is right-to-left (the operating system is in Arabic or Hebrew languages). The scrollbar is then not on the right, but on the left, and then clientLeft also includes the scrollbar width.

In that case, clientLeft would be not 25, but with the scrollbar width 25 + 16 = 41.

Here’s the example in hebrew:

clientWidth/Height

These properties provide the size of the area inside the element borders.

They include the content width together with paddings, but without the scrollbar: