window.getComputedStyle

シンタックス

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

element

この Element の計算したスタイルを取得します。

pseudoElt

マッチさせたい疑似要素を指定する文字列です。通常要素には空文字列を指定してください。

返り値 style は、CSSSTyleDeclaration オブジェクトです。この値は要素のスタイルが変わると自動で更新します。

Throws

TypeError

渡されたオブジェクトが Element ではない。もしくは pseudoElt が適切な疑似要素セレクタではないか::part()または::slotted()です。

Note: 適切な疑似要素とは構文的に適切であることを指します。例えば "::unsupported" は疑似要素としてはサポートされていませんが、適切と判断されます。

例

この例では、<p> の要素のスタイルを設定してから、getComputedStyle() を使用してそれらのスタイルを取得し、それらを<p>のテキストコンテンツに出力します。

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

let para = document.querySelector('p');
let compStyles = window.getComputedStyle(para);
para.textContent = 'My computed font-size is ' +
    compStyles.getPropertyValue('font-size') +
    ',\nand my computed line-height is ' +
    compStyles.getPropertyValue('line-height') +
    '.';

Result

詳細

The returned object is the same CSSStyleDeclaration type as the object returned from the element's 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 Element. non-elements, like a Text node, will throw an error.

defaultView

<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>

• The returned CSSStyleDeclaration object contains active values for CSS property longhand names. For example, border-bottom-width instead of the border-width and border shorthand property names. It is safest to query values with only longhand names like font-size. Shorthand names like font will not work with most browsers.
• CSS property values may be accessed using the getPropertyValue(propName) API or by indexing directly into the object 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.

仕様

ブラウザ互換性

参照

• window.getDefaultComputedStyle
• resolved value