Window: getComputedStyle() method

{{APIRef("CSSOM")}}

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.

Individual CSS property values are accessed through APIs provided by the object, or by indexing with CSS property names.

Syntax

getComputedStyle(element)
getComputedStyle(element, pseudoElt)

Parameters

element
- : The {{DOMxRef("Element")}} for which to get the computed style.
pseudoElt {{optional_inline}}
- : A string specifying the pseudo-element to match. Omitted (or null) for real elements.

Return value

A live {{DOMxRef("CSSStyleDeclaration")}} object, which updates automatically when the element’s styles are changed.

Exceptions

{{JSxRef("TypeError")}}
- : If the passed object is not an {{DOMxRef("Element")}} or the pseudoElt is not a valid pseudo-element selector or is {{CSSxRef("::part", "::part()")}} or {{CSSxRef("::slotted", "::slotted()")}} .
  
  [!NOTE] Valid pseudo-element selector refers to syntactic validity, e.g. ::unsupported is considered valid, even though the pseudo-element itself is not supported. Additionally, the latest W3 standard explicitly supports only ::before and ::after, while the CSS WG draft does not restrict this value. Browser compatibility may vary.

Examples

In this example we style a {{HTMLElement("p")}} element, then retrieve those styles using getComputedStyle(), and print them into the text content of the <p>.

HTML

<p>Hello</p>

CSS

p {
  width: 400px;
  margin: 0 auto;
  padding: 20px;
  font: 2rem/2 sans-serif;
  text-align: center;
  background: purple;
  color: white;
}

JavaScript

const para = document.querySelector("p");
const compStyles = window.getComputedStyle(para);
para.textContent =
  `My computed font-size is ${compStyles.getPropertyValue("font-size")},\n` +
  `and my computed line-height is ${compStyles.getPropertyValue(
    "line-height",
  )}.`;

Result

{{EmbedLiveSample('Examples', '100%', '240px')}}

Description

The returned object is the same {{DOMxRef("CSSStyleDeclaration")}} type as the object returned from the element’s {{DOMxRef("HTMLElement/style", "style")}} property. However, the two objects have different purposes:

The object from getComputedStyle is read-only, and should be used to inspect the element’s style — including those set by a <style> element or an external stylesheet.
The element.style object should be used to set styles on that element, or inspect styles directly added to it from JavaScript manipulation or the global style attribute.

The first argument must be an {{domxref("Element")}} . Non-elements, like a {{DOMxRef("Text")}} node, will throw an error.

defaultView

In many code samples, getComputedStyle is used from the {{DOMxRef("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 a combination of folks not wanting to write a testing spec for window and making an API that was also usable in Java.

Use with pseudo-elements

getComputedStyle can pull style info from pseudo-elements (such as ::after, ::before, ::marker, ::line-marker — see the pseudo-element spec).

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

<h3>Generated content</h3>

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

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

Notes

The returned {{DOMxRef("CSSStyleDeclaration")}} object contains active values for CSS property longhand names as well as shorthand names. For example, the returned object contains entries for {{cssxref("border-bottom-width")}} in addition to the {{cssxref("border-width")}} and {{cssxref("border")}} shorthand property names. You can query values with longhand names like {{cssxref("font-size")}} as well as shorthand names like {{cssxref("font")}} .
CSS property values may be accessed using the {{DOMxRef("CSSStyleDeclaration.getPropertyValue", "getPropertyValue(propName)")}} method or by indexing directly into the object using array or dot notation such as obj['z-index'] or obj.zIndex.
The values returned by getComputedStyle are resolved values. These are usually the same as CSS 2.1’s computed values, but for some older properties like width, height, or padding, they are instead the same as used values. Originally, CSS 2.0 defined the computed values as the “ready to be used” final values of properties after cascading and inheritance, but CSS 2.1 redefined them as pre-layout, and used values as post-layout. For CSS 2.0 properties, getComputedStyle returns the old meaning of computed values, now called used values. An example difference between pre- and post-layout values includes the resolution of percentages for width or height, as those will be replaced by their pixel equivalent only for used values.
Returned values are sometimes deliberately inaccurate. To avoid the “CSS History Leak” security issue, browsers may lie about the computed styles for a visited link, returning values as if the user never visited the linked URL. See Plugging the CSS history leak and Privacy-related changes coming to CSS :visited for examples of how this is implemented.
During CSS transitions, 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 a containing block of height:100px, Firefox’s computed style for top returns 70px, as 100 − 30 = 70.
For compatibility reasons, serialized color values are expressed as rgb() colors if the alpha channel value is exactly 1, and rgba() colors otherwise. In both cases, legacy syntax is used, with commas as separators (for example rgb(255, 0, 0)).

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

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