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Inject New CSS Rules

If you need to change the style of an element with JavaScript, it's typically better to change a class name and have the CSS already on the page take effect and change the style. However, there are exceptions to every rule. For instance, you might want to programmatically change the a pseudo class (e.g. :hover). You can't do that through JavaScript for the same reason inline style="" attributes can't change pseudo classes.

You'll need to inject a new <style> element onto the page with the correct styles in it. Best to inject it at the bottom of the page so it overrides your CSS above it. Easy with jQuery:

function injectStyles(rule) {
  var div = $("<div />", {
    html: '&shy;<style>' + rule + '</style>'
  }).appendTo("body");    
}

Usage

injectStyles('a:hover { color: red; }');

Demo

More Information

Comments

  1. Patrick Neal
    Permalink to comment#

    Good post, but wouldn’t it be preferable to simply add a class that solely changes the hover state? For instance add a class of .hoverChange, and in the CSS just add a rule like .hoverChange:hover{ color: red; }?

    • Possibly. But you just can’t know every possible situation. Like I mentioned above, what if you need to programmatically calculate what the style will be?

  2. Isn’t a style element supposed to go in head, since it’s metadata content and all. That is, unless the scope attribute is present… See style element

    • Yeah it is, but there’s never been a browser where it didn’t work no matter where you put it. I think think it’s like 0.0001% safer to use body instead of head. Come to think of it, I think the safest possible way is to target the first script tag it can find (there must be one somewhere, since this is JavaScript running) and insert the style element after it.

  3. Dr. Clue

    This one is actually even a little more fun as you can create pretty much any CSS you could create in a stylesheet, using any of the selectors the browser supports, and they can be

    document.styleSheets[0].insertRule(“body {background:red;}”, 0);
    document.styleSheets[0].deleteRule(0)

    The 0 is the position index. I actually have a class scribbled up for this stuff that does things like detect and map standards CSS to vendor prefixed , produces re-usable CSS for CSS3D stuff like carousels , cubes , pyramids (with textures),manipulates keyframe rules etc. It will even tell me all the classes that effect an element , elements or even elements matching a regular expression query. The only reason this other stuff is mentioned is simply to point out just how much can actually be done working at the rule level

  4. Elise Chant
    Permalink to comment#

    It would be better to give the div an id so you can dispose of it easily and instantiate it only once.

    injectStyles: function(id, rule) {
        var $div = "<div id='"+ id +"'><style>"+ rule + "</style></div>";
    
        if ($('body ' + id).length === 0) {
            $($div).appendTo('body');
        }
    }
    
  5. Agreed. I’m actually quite sure that jQuery popularized a pretty bad approach to CSS styling (inline styles). It’s slow and doesn’t make sense if you think about it (applying CSS to a class doesn’t persist to new elements of that class).

    CSS injection really seems to be the way to go.

    I wrote veinjs just for this purpose:

    https://github.com/israelidanny/veinjs

  6. Garlaro
    Permalink to comment#

    Thanks, very helpful!
    I needed a “no framework” function, so here it is (based on all the above):

    http://stackoverflow.com/a/26230472/2752979

    code and demo

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