Register and enqueue conditional (browser-specific) javascript files?

Question

The WP.SE community has always advised the use of wp_register_script and wp_enqueue_script for adding scripts in a theme/template (and likewise, wp_register_style and wp_enqueue_style for stylesheets).

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This is how I register and enqueue my scripts...

First, I add something like this in functions.php

add_action('init','wpse54189_register_script');
function wpse54189_register_script(){

    //Register scripts
    wp_enqueue_script( 'jquery' );
    wp_register_script( 'aahan_bootstrap_transition', get_template_directory_uri().'/js/bootstrap-transition.js');
    wp_register_script( 'aahan_bootstrap_carousel', get_template_directory_uri().'/js/bootstrap-carousel.js', array('aahan_bootstrap_transition'));    
    wp_register_script( 'wpse54189_ajax_comment', get_template_directory_uri().'/js/no-reload-comments.js');
}

then call it in a template file (say, header.php) like this

<?php wp_enqueue_script( 'aahan_bootstrap_carousel' ); ?>
<?php wp_enqueue_script( 'wpse54189_ajax_comment' ); ?>

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Now, coming to the point, how do I register and enqueue "conditional JavaScript files" that are there to be recognized by specific browsers? For example:

<!--[if lt IE 9]>
<script src="<?php echo get_template_directory_uri(); ?>/js/html5.js" type="text/javascript"></script>
<![endif]-->

How do I register and enqueue this properly?

PS: I use the Reddle theme, developed by the theme wranglers at Automattic. And the theme's header.php directly uses the just aforementioned code -- i.e. it's not enqueued. So, does that mean, it's the only way to do it?

Answer 1

WP_Scripts and WP_Styles classes are behind wp_enqueue_script and wp_enqueue_style functions. If you take a look at classes implementation (scripts and styles) then you will see that WP_Scripts class doesn't support any kind of conditional scripts, but! you can see that WP_Styles does! The problem is that wp_enqueue_style doesn't allow you to setup condition.

So we have to make a small hack:

add_filter( 'wp_enqueue_scripts', 'wpse2345_enqueue_scripts' );
function wpse2345_enqueue_scripts() {
    wp_enqueue_style( 'mystyle', 'url/of/my/style.css', array(), '1.0.0' );

    global $wp_styles;
    $wp_styles->registered['mystyle']->add_data( 'conditional', 'lt IE 9' );
}

Such hack become possible, because all registered styles are stored in registered field of WP_Styles class. Each of registered style is an object of _WP_Dependency class, which allow you to add extra data.

Unfortunately this hack is not working for scripts.

Additional Information:
I was actually going through the code in Aaron Campbell's Essence Theme last night and noticed that he was calling both a browser conditional script and style.

/**
 * @var WP_Scripts
 */
global $wp_scripts;
// Conditionally load this only for IE < 9
$wp_scripts->add_data( 'html5', 'conditional', 'lt IE 9' );

/**
 * @var WP_Styles
 */
global $wp_styles;
// Conditionally load this only for IE < 8
$wp_styles->add_data( 'blueprint-ie', 'conditional', 'lt IE 8' );

There is also a ticket and patch but it's not in core yet. Obviously the conditional script won't work without the patch but one thing to note is that you can use the add_data method directly inside your function that is attached to the wp_enqueue_scripts action.