How Can I Retrieve the Initially Defined CSS Property Values (Including Percentage and EM Units)?

Get CSS Property Values as Initially Defined (Percent/EM/PX)

In web development, it can be crucial to retrieve the original CSS property value of an element, regardless of any subsequent modifications. This is especially true when working with units like percent or em, as computed values often provide px values instead.

Problem Definition

The problem arises when using functions like getComputedStyle or jQuery's css() to obtain the current CSS property value. These methods return the value in pixels, even if initially set in other units.

Solution: getMatchedStyle

To address this issue, a custom JavaScript function called getMatchedStyle can be employed. This function iterates through the element's CSS rules, prioritizing element styles and important rules. It then returns the initial value as defined in the CSS rules.

Code Example

function getMatchedStyle(elem, property) {
  // Check for element-defined property (highest priority)
  var val = elem.style.getPropertyValue(property);

  // If important, return immediately
  if (elem.style.getPropertyPriority(property)) return val;

  // Get matched CSS rules in descending priority order
  var rules = getMatchedCSSRules(elem);

  // Iterate through rules backwards to ensure correct priority
  for (var i = rules.length; i--;) {
    var r = rules[i];

    // Check if rule is important
    var important = r.style.getPropertyPriority(property);

    // Update value only if not set or if important rule encountered
    if (val == null || important) {
      val = r.style.getPropertyValue(property);

      // Return if important rule encountered
      if (important) break;
    }
  }

  return val;
}

Example Usage

Consider the following HTML and CSS:

<div class="b">first</div>
<div>
<p>The following JavaScript code can be used to retrieve the initial widths of the divs:</p>
<pre class="brush:php;toolbar:false">var d = document.querySelectorAll('div');

for (var i = 0; i <p><strong>Output</strong></p><pre class="brush:php;toolbar:false">div 1:  100px
div 2:  50%

By using the getMatchedStyle function, we can accurately retrieve the CSS property values as originally defined, regardless of whether they were modified by subsequent styles.