Window.getComputedStyle()

Summary

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

Syntax

var style = window.getComputedStyle(element[, pseudoElt]);

element

The Element for which to get the computed style.

pseudoElt Optional

A string specifying the pseudo-element to match. Must be omitted (or null) for regular elements.

The returned style is a CSSStyleDeclaration object.

Example

var elem1 = document.getElementById("elemId");
var style = window.getComputedStyle(elem1, null);

// this is equivalent:
// var style = document.defaultView.getComputedStyle(elem1, null);

<style>
 #elem-container{
   position: absolute;
   left:     100px;
   top:      200px;
   height:   100px;
 }
</style>

<div id="elem-container">dummy</div>
<div id="output"></div>  

<script>
  function getTheStyle(){
    var elem = document.getElementById("elem-container");
    var theCSSprop = window.getComputedStyle(elem,null).getPropertyValue("height");
    document.getElementById("output").innerHTML = theCSSprop;
   }
  getTheStyle();
</script>

function dumpComputedStyles(elem,prop) {

  var cs = window.getComputedStyle(elem,null);
  if (prop) {
    dump("    "+prop+" : "+cs.getPropertyValue(prop)+"\n");
    return;
  }
  var len = cs.length;
  for (var i=0;i<len;i++) {
 
    var style = cs[i];
    dump("    "+style+" : "+cs.getPropertyValue(style)+"\n");
  }

}

Description

The returned object is of the same type that the object returned from the element's style property; however, the two objects have different purposes. The object returned from getComputedStyle is read-only and can be used to inspect the element's style (including those set by a <style> element or an external stylesheet). The elt.style object should be used to set styles on a specific element.

The first argument must be an Element (passing a non-Element Node, like a #text Node, will throw an error). Starting in Gecko 1.9.2 (Firefox 3.6 / Thunderbird 3.1 / Fennec 1.0), returned URL values now have quotes around the URL, like this: url("https://foo.com/bar.jpg").

defaultView

In many code samples online, getComputedStyle is used from the document.defaultView object. In nearly all cases, this is needless, as getComputedStyle exists on the window object as well. It's likely the defaultView pattern was some combination of (1) folks not wanting to write a spec for window and (2) making an API that was also usable in Java. However, there is a single case where the defaultView's method must be used: when using Firefox 3.6 to access framed styles.

Use with pseudo-elements

getComputedStyle can pull style info from pseudo-elements (for example, ::after, ::before, ::marker, ::line-marker—see spec here).

<style>
 h3::after {
   content: ' rocks!';
 }
</style>

<h3>generated content</h3> 

<script>
  var h3       = document.querySelector('h3'), 
      result   = getComputedStyle(h3, ':after').content;

  console.log('the generated content is: ', result); // returns ' rocks!'
</script>

Notes

The values returned by getComputedStyle are known as resolved values. These are usually the same as the CSS 2.1 computed values, but for some older properties like width, height or padding, they are instead the used values. Originally, CSS 2.0 defined the computed values to be the "ready to be used" final values of properties after cascading and inheritance, but CSS 2.1 redefined computed values as pre-layout, and used values as post-layout. For CSS 2.0 properties, the getComputedStyle function returns the old meaning of computed values, now called used values. An example of difference between pre- and post-layout values includes the resolution of percentages that represent the width or the height of an element (also known as its layout), as those will be replaced by their pixel equivalent only in the used value case.

The returned value is, in certain known cases, expressly inaccurate by deliberate intent. In particular, to avoid the so called CSS History Leak security issue, browsers may expressly "lie" about the used value for a link and always return values as if a user has never visited the linked site. See https://blog.mozilla.com/security/2010/03/31/plugging-the-css-history-leak/ and https://hacks.mozilla.org/2010/03/privacy-related-changes-coming-to-css-vistited/ for details of the examples of how this is implemented. Most other modern browsers have applied similar changes to the application of pseudo-selector styles and the values returned by getComputedStyle.

During a CSS transition, getComputedStyle returns the original property value in Firefox, but the final property value in WebKit.

In Firefox, properties with the value auto return the used value, not the value auto. So if you apply top:auto; and bottom:0; on an element with height:30px and its containing block is height:100px;, upon requesting the computed style for top, Firefox will return top:70px, as 100px-30px=70px.

Browser compatibility

Specification

• DOM Level 2 Style: getComputedStyle
• CSS Object Model specification

See also

• window.getDefaultComputedStyle
• resolved value