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How Can I Retrieve the Initially Defined CSS Property Values (Including Percentage and EM Units)?
How Can I Retrieve the Initially Defined CSS Property Values (Including Percentage and EM Units)?
Dec 15, 2024 pm 02:49 PM
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>
The following JavaScript code can be used to retrieve the initial widths of the divs:
var d = document.querySelectorAll('div');
for (var i = 0; i < d.length; ++i) {
console.log("div " + (i + 1) + ": " + getMatchedStyle(d[i], 'width'));
}
Output
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.
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