Changing width property of a :before css selector using JQuery

I have a page that has an image in it and I styled it using :before CSS selectors.
The image is dynamic so it hasn't a fixed width; So I need to set :before rule's width dynamically.
I want do it in client side using JQuery.
Assume this:

.column:before{
	width: 300px;
	float: left;
	content: "";
	height: 430px;
}

.column{
	width: 500px;
	float: right;
	padding: 5px;
	overflow: hidden;
	text-align: justify;
}

How Can I only change the width property of class with :before selector (and not one without it) using JQuery?

I don't think there's a jQuery-way to directly access the pseudoclass' rules, but you could always append a new style element to the document's head like:

$('head').append('<style>.column:before{width:800px !important;}</style>');

See a live demo here

I also remember having seen a plugin that tackles this issue once but I couldn't find it on first googling unfortunately.

Pseudo elements are part of the shadow DOM and can not be modified (but can have their values queried).

However, sometimes you can get around that by using classes, for example.

jQuery

$('#element').addClass('some-class');

CSS

.some-class:before {
    /* change your properties here */
}

This may not be suitable for your query, but it does demonstrate you can achieve this pattern sometimes.

To get a pseudo element's value, try some code like...

var pseudoElementContent = window.getComputedStyle($('#element')[0], ':before')
  .getPropertyValue('content')

Pseudo-elements are not part of the DOM, so they can't be manipulated using jQuery or Javascript.

But as pointed out in the accepted answer, you can use the JS to append a style block which ends of styling the pseudo-elements.

The answer should be Jain. You can not select an element via pseudo-selector, but you can add a new rule to your stylesheet with insertRule.

I made something that should work for you:

var addRule = function(sheet, selector, styles) {
    if (sheet.insertRule) return sheet.insertRule(selector + " {" + styles + "}", sheet.cssRules.length);
    if (sheet.addRule) return sheet.addRule(selector, styles);
};

addRule(document.styleSheets[0], "body:before", "content: 'foo'");

http://fiddle.jshell.net/MDyxg/1/

To be super-cool (and to answer the question really) I rolled it out again and wrapped this in a jQuery-plugin (however, jquery is still not required!):

/*!
 * jquery.addrule.js 0.0.1 - https://gist.github.com/yckart/5563717/
 * Add css-rules to an existing stylesheet.
 *
 * @see http://stackoverflow.com/a/16507264/1250044
 *
 * Copyright (c) 2013 Yannick Albert (http://yckart.com)
 * Licensed under the MIT license (http://www.opensource.org/licenses/mit-license.php).
 * 2013/05/12
 **/

(function ($) {

    window.addRule = function (selector, styles, sheet) {

        styles = (function (styles) {
            if (typeof styles === "string") return styles;
            var clone = "";
            for (var p in styles) {
                if (styles.hasOwnProperty(p)) {
                    var val = styles[p];
                    p = p.replace(/([A-Z])/g, "-$1").toLowerCase(); // convert to dash-case
                    clone += p + ":" + (p === "content" ? '"' + val + '"' : val) + "; ";
                }
            }
            return clone;
        }(styles));
        sheet = sheet || document.styleSheets[document.styleSheets.length - 1];

        if (sheet.insertRule) sheet.insertRule(selector + " {" + styles + "}", sheet.cssRules.length);
        else if (sheet.addRule) sheet.addRule(selector, styles);

        return this;

    };

    if ($) $.fn.addRule = function (styles, sheet) {
        addRule(this.selector, styles, sheet);
        return this;
    };

}(window.jQuery));

The usage is quite simple:

$("body:after").addRule({
    content: "foo",
    color: "red",
    fontSize: "32px"
});

// or without jquery
addRule("body:after", {
    content: "foo",
    color: "red",
    fontSize: "32px"
});

https://gist.github.com/yckart/5563717

You may try to inherit property from the base class:

var width = 2;
var interval = setInterval(function () {
    var element = document.getElementById('box');
    width += 0.0625;
    element.style.width = width + 'em';
    if (width >= 7) clearInterval(interval);
}, 50);

.box {
    /* Set property */
    width:4em;
    height:2em;
    background-color:#d42;
    position:relative;
}
.box:after {
    /* Inherit property */
    width:inherit;
    content:"";
    height:1em;
    background-color:#2b4;
    position:absolute;
    top:100%;
}

<div id="box" class="box"></div>

One option is to use an attribute on the image, and modify that using jQuery. Then take that value in CSS:

HTML (note I'm assuming .cloumn is a div but it could be anything):

<div class="column" bf-width=100 >
    <img src="..." />
</div>

jQuery:

// General use:
$('.column').attr('bf-width', 100);
// With your image, along the lines of:
$('.column').attr('bf-width', $('img').width());

And then in order to use that value in CSS:

.column:before {
    content: attr(data-content) 'px';
    /* ... */
}

This will grab the attribute value from .column, and apply it on the before.

Sources: CSS attr (note the examples with before), jQuery attr.

As Boltclock states in his answer to Selecting and manipulating CSS pseudo-elements such as ::before and ::after using jQuery

> Although they are rendered by browsers through CSS as if they were like other real DOM elements, pseudo-elements themselves are not part of the DOM, and thus you can't select and manipulate them with jQuery.

Might just be best to set the style with jQuery instead of using the pseudo CSS selector.