How to access CSS generated content with JavaScript

Question

I generate the numbering of my headers and figures with CSS's counter and content properties:

img.figure:after {
  counter-increment: figure;
  content: "Fig. " counter(section) "." counter(figure);
}

This (appropriate browser assumed) gives a nice labelling "Fig. 1.1", "Fig. 1.2" and so on following any image.

Question: How can I access this from Javascript? The question is twofold in that I'd like to access either the current value of a certain counter (at a certain DOM node) or the value of the CSS generated content (at a certain DOM node) or, obviously, both information.

Background: I'd like to append to links back-referencing to figures the appropriate number, like this:

<a href="#fig1">see here</h>
------------------------^ " (Fig 1.1)" inserted via JS

As far as I can see, it boils down to this problem: I could access content or counter-increment via getComputedStyle:

var fig_content = window.getComputedStyle(
                    document.getElementById('fig-a'),
                    ':after').content;

However, this is not the live value, but the one declared in the stylesheet. I cannot find any interface to access the real live value. In the case of the counter, there isn't even a real CSS property to query.

Edit: Digging deeper and deeper through the DOM specs, I found the DOM Level 2 Style Counter interface. This seems to a) allow access to the current counter value and b) be implemented in Firefox, at least. However, I have no idea on how to use it. My current approach died tragically after this Firebug output:

// should return a DOM 2 Counter interface implementation...
window.getComputedStyle(fig_a_element, ':after')
      .getPropertyCSSValue("counter-increment")[0]
      .getCounterValue();

[Exception... "Modifications are not allowed for this document" code: "7"
 nsresult: "0x80530007 (NS_ERROR_DOM_NO_MODIFICATION_ALLOWED_ERR)"
 location: "http://localhost/countertest.html Line: 71"]

Any idea, how this could be brought to life would be highly appreciated.

Edit 2: Apparently I misinterpreted the Counter object of DOM Level 2 Style. It, too, has no property to return the current counter value. This makes the above approach invalid.

New approach: Is there a possibility to read the content of a pseudo-element via the DOM? That is, can I select the pseudo-element (treeWalker comes to mind) and then get its nodeValue? (If you start to type 'jQuery' now, please reconsider to change that term into 'Sizzle'...)

Answer 1

I cannot find any interface to access the real live value.

If you can't get the value from window.getComputedStyle, it seems that it would be impossible.

More importantly, while it can do it, I think this might be abusing CSS since you're breaking the barrier between content and presentation at this point.

Check out this related question What are good uses of css “Content” property?

Answer 2

I cannot find any interface to access the real live value. [of the counter]

Yeah. I don't think there is one. Sorry.

The only thing I can think of would be to go through every element (including its :before/:after pseudo-elements) before the element in the document, looking for counters and adding up how many there are.

Obviously that's hideous. If you're going to try to reproduce the browser's own counter mechanism it would probably be easier (and much more compatible, given IE<=7's lack of counter/content support) to just replace it with your own script-based counters. eg. something along the lines of:

<a href="#prettypicture">this</a>

<div class="counter level=0">...</div>
<img id="prettypicture" class="counter level=1" alt="ooo, pretty"/>

window.onload= function() {
    var counters= Node_getElementsByClassName(document.body, 'counter');
    var indices= [];
    for (var counteri= 0; counteri<counters.length; counteri++) {
        var counter= counters[counteri];

        var level= Element_getClassArgument(counter, 'level');
        while (indices.length<=level)
            indices.push(0);
        indices[level]++;
        indices= indices.slice(level+1);
        var text= document.createTextNode('Figure '+indices.join('.'));
        counter.parentNode.insertBefore(text, counter.nextSibling);

        if (counter.id!=='') {
            for (var linki= document.links.length; linki-->0;) {
                var link= document.links[i];
                if (
                    link.hostname===location.hostname && link.pathname===location.pathname &&
                    link.search===location.search && link.hash==='#'+counter.id
                ) {
                    var text= document.createTextNode('('+indices.join('.')+')');
                    link.parentNode.insertBefore(text, link.nextSibling);
                }
            }
        }
    }
};

Answer 3

I would port your css to Javascript, this enables you to get the figure caption and also you get greater browser coverage. Using jQuery you'd do something like this:

$(function() {
  var section = 0;
  $(".section").each(function() {
    section++;
    var figure = 0;
    $(this).find("img.figure").each(function() {
      figure++;
      var s = "Fig. " + section + "." + figure;
      $(this).attr({alt:s}).after(s);
    });
  });
});

Then you could do:

<div class="section">blabla <img id="foo" src="http://www.example.com/foo.jpg"&gt;&lt;/div&gt;
<p>Lorem ipsum dolor sit amet <a href="#foo" class="figurereference">see here</a></p>

<script type="text/javascript">
  $(function() {
    $("a.figurereference").each(function() {
      var selector = $(this).attr("href");
      var $img = $(selector);
      var s = $(this).text() + " (" + $img.attr("alt") + ")";
      $(this).text(s);
    });
  });
</script>

Though I agree that doing this using CSS would be very neat.

Answer 4

read this:

• http://www.w3.org/TR/CSS2/generate.html#propdef-content

Generated content does not alter the document tree. In particular, it is not fed back to the document language processor (e.g., for reparsing).

---

• https://developer.mozilla.org/en/CSS/Getting_Started/Content

Content specified in a stylesheet does not become part of the DOM.

so for this reason the getComputedStyle will never work in this case; i think the only way, is to perform a classic loop through as someone has described below!