body_class

When it fires

WordPress applies body_class when the theme calls body_class() on the tag. Classic themes do this in header.php and block themes get it automatically. It only runs on front-end pages.

The main query has run by then, so you can use conditional tags to decide which classes to add.

Parameters

Parameters

$classesstring[]	An array of body class names.
$css_classstring[]	An array of additional class names added to the body.

What to return

The array of class names. Add to it or remove from it, then return it.

Example: Add the page slug as a body class

add_filter( 'body_class', function ( $classes ) {
	if ( is_singular() ) {
		$slug = get_post_field( 'post_name', get_queried_object_id() );
		if ( $slug ) {
			$classes[] = 'slug-' . sanitize_html_class( $slug );
		}
	}
	return $classes;
} );

Good to know

• Always return the array. Returning nothing causes an error when the classes are printed.
• The admin area has its own filter, admin_body_class, which works with a space-separated string, not an array.