Adding Filter Conditionally Per Page ID

Question

I've created a function to add a class to the body, however if should only apply to a single page ID. This normally wouldn't be a problem, but it is breaking the layout for other pages.

Filter Function

add_filter('body_class','add_blog_to_body_class');

function add_blog_to_body_class($classes) {
    $classes[] = 'blog';
        return $classes;
}

My Condition (that isn't working)

function add_blog_to_body_class($classes) {
    $classes[] = 'blog';
    if ( is_page('ID') ) {
        return $classes;
    }
}

I've wrapped the entire function, the declaration, the add_filter hook, and everything without luck.

How can I have this function work on a single page ID?

Answer 1

It's important to always return the value in filter callbacks, otherwise it's like adding the __return_null() callback.

Here's an example that adds the blog class only to page with ID 40:

add_filter( 'body_class', 'add_blog_to_body_class' );

function add_blog_to_body_class( $classes )
{
    if ( is_page( 40 ) )
        $classes[] = 'blog';

    return $classes;
}