Add featured image to wp_nav_menu items - wordpress

Question

This is a self Q&A.
How do you modify the text/html that appears in the output of a wp_nav_menu? For example, I wanted to add the featured image for pages and categories.
You see examples of doing this with a custom walker, but the code is very complex to do for small changes. Surely there is a way to do it with a filter?

Answer 1

This is the code I came up with thanks to some help from a Wordpress StackOverflow answer that I can't find anymore (please comment with a link if you find it).
First you need to add the filter to the specific menu (you could add it to all menus if you want - just use the add_filter line by itself).
// Add filter to specific menus
add_filter('wp_nav_menu_args', 'add_filter_to_menus');
function add_filter_to_menus($args) {
// You can test agasint things like $args['menu'], $args['menu_id'] or $args['theme_location']
if( $args['theme_location'] == 'header_menu') {
add_filter( 'wp_setup_nav_menu_item', 'filter_menu_items' );
}
return $args;
}
Then you need to build out the code to get the post or category ID from the $item object passed to the filter. It's not as easy as you'd expect, as $item doesn't contain the underlying post/category ID, just the menu item ID. So I use the URL's to do a reverse lookup of the IDs.
This won't work for tags used in a menu, or custom taxonomys. I only needed it for categories, so this is all I built.
// Filter menu
function filter_menu_items($item) {
if( $item->type == 'taxonomy') {
// For category menu items
$cat_base = get_option('category_base');
if( empty($cat_base) ) {
$cat_base = 'category';
}
// Get the path to the category (excluding the home and category base parts of the URL)
$cat_path = str_replace(home_url().'/'.$cat_base, '', $item->url);
// Get category and image ID
$cat = get_category_by_path($cat_path, true);
$thumb_id = get_term_meta($cat->term_id, '_term_image_id', true); // I'm using the 'Simple Term Meta' plugin to store an attachment ID as the featured image
} else {
// Get post and image ID
$post_id = url_to_postid( $item->url );
$thumb_id = get_post_thumbnail_id( $post_id );
}
if( !empty($thumb_id) ) {
// Make the title just be the featured image.
$item->title = wp_get_attachment_image( $thumb_id, 'poster');
}
return $item;
}
And then you want to remove the filter that you applied at the beginning, so that the next menu processed doesn't use the same HTML as defined above in filter_menu_items().
// Remove filters
add_filter('wp_nav_menu_items','remove_filter_from_menus', 10, 2);
function remove_filter_from_menus( $nav, $args ) {
remove_filter( 'wp_setup_nav_menu_item', 'filter_menu_items' );
return $nav;
}

Answer 2

Modified Drew Baker answer. It works without plugins, also if there is no category with current slug it checks for woocommerce product category ('product_cat').
functions.php
// Add filter to specific menus
add_filter('wp_nav_menu_args', 'add_filter_to_menus');
function add_filter_to_menus($args) {
// You can test agasint things like $args['menu'], $args['menu_id'] or $args['theme_location']
if( $args['theme_location'] == 'menu-header') {
add_filter( 'wp_setup_nav_menu_item', 'filter_menu_items' );
}
return $args;
}
// Filter menu
function filter_menu_items($item) {
if( $item->type == 'taxonomy') {
// Get category and image ID
$slug = pathinfo( $item->url, PATHINFO_BASENAME );
$cat = get_term_by( 'slug', $slug, 'category' );
// If there is no standard category try getting product category
if( !$cat ) {
$cat = get_term_by( 'slug', $slug, 'product_cat' );
}
$thumb_id = get_term_meta($cat->term_id, 'thumbnail_id', true);
} else {
// Get post and image ID
$post_id = url_to_postid( $item->url );
$thumb_id = get_post_thumbnail_id( $post_id );
}
if( !empty($thumb_id) ) {
// Make the title just be the featured image.
$item->title = wp_get_attachment_image( $thumb_id, 'poster');
// Display image + title example
// $item->title = wp_get_attachment_image( $thumb_id, 'poster').$item->title;
}
return $item;
}
// Remove filters
add_filter('wp_nav_menu_items','remove_filter_from_menus', 10, 2);
function remove_filter_from_menus( $nav, $args ) {
remove_filter( 'wp_setup_nav_menu_item', 'filter_menu_items' );
return $nav;
}