Question

For example I want to find all elements that have computed style position: fixed;. How to do it without making much load on the CPU ?

Is iterating every getElementsByTagName('*') and then doing for loop the only way ?

Answer 1

Instead of selecting all (*) elements, and use getComputedStyle + getPropertyValue, you can follow the following steps:

• Loop through all CSS rules (via document.styleSheets ^[1]) and take the selectors which contains position: fixed.

• Select all elements whose style attribute contains position: fixed.

• Use document.querySelectorAll to select all elements which match the selector.

  • Test whether window.getComputedStyle(elem, null).getPropertyValue('position') equals fixed to filter elements which are not at a fixed position (possibly overridden through a more specific selector, or !important).
  • If it matches, push the element in an array

• At this point, you have an array containing all position: fixed elements.

^{[1] The external stylesheets have to be located at the same origin, because of the Same origin policy.}

Code (small demo: http://jsfiddle.net/GtXpw/):

//[style*=..] = attribute selector
var possibilities = ['[style*="position:fixed"],[style*="position: fixed"]'],
    searchFor = /\bposition:\s*fixed;/,
    cssProp = 'position',
    cssValue = 'fixed',
    styles = document.styleSheets,
    i, j, l, rules, rule, elem, res = [];

for (i=0; i<styles.length; i++) {
    rules = styles[i].cssRules;
    l = rules.length;
    for (j=0; j<l; j++) {
        rule = rules[j];
        if (searchFor.test(rule.cssText)) {
            possibilities.push(rule.selectorText);
        }
    }
}
possibilities = possibilities.join(',');
possibilities = document.querySelectorAll(possibilities);
l = possibilities.length;
for (i=0; i<l; i++) {
   elem = possibilities[i];
   // Test whether the element is really position:fixed
   if (window.getComputedStyle(elem, null).getPropertyValue(cssProp) === cssValue) {
       res.push(elem);
   }
}
res; //<-- = Array containing all position:fixed elements