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Summary

The Window.getComputedStyle() method returns an object that reports the values of all CSS properties of an element after applying active stylesheets and resolving any basic computation those values may contain.  Individual CSS property values are accessed through APIs provided by the object or by simply indexing with CSS property names.

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.
Note: Prior to Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1), the pseudoElt parameter was required. No other major browser required this parameter be specified if null. Gecko has been changed to match the behavior of other browsers.

The returned style is a live CSSStyleDeclaration object, which updates itself automatically when the element's style is changed.

Example

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

// this is equivalent to:
// 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) {
    console.log(prop+" : "+cs.getPropertyValue(prop));
    return;
  }
  var len = cs.length;
  for (var i=0;i<len;i++) {
 
    var style = cs[i];
    console.log(style+" : "+cs.getPropertyValue(style));
  }

}

Description

The returned object is of the same CSSStyleDeclaration type as 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("http://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'); 
  var result = getComputedStyle(h3, ':after').content;

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

Notes

The returned CSSStyleDeclaration object will contain active values for all supported CSS property longhand names.  An example longhand name is border-bottom-width with border-width and border being example shorthand property names. It is safest to query for values using only longhand names like font-size.  Querying with shorthand names like font will not work with the majority of browsers.

The CSS specifications also permit using camel-cased names such as fontSize or paddingTop.

The CSS property values may be accessed using the getPropertyValue(propName) API or by indexing directly into the object such as cs[' z-index'] or cs.zIndex.

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 http://blog.mozilla.com/security/2010/03/31/plugging-the-css-history-leak/ and http://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

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) 9 (Yes) (Yes)
pseudo-element support (Yes) ? (Yes) 9 15 (Yes)
Feature Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) WP7 Mango (Yes) (Yes)
pseudo-element support ? ? ? No support ? ?

Specification

See also