Proof of concept for how to add new fields to nav_menu_item posts in the WordPress menu editor.

nav-menu-item-custom-fields.php

<?php
/**
 * Proof of concept for how to add new fields to nav_menu_item posts in the WordPress menu editor.
 * @author Weston Ruter (@westonruter), X-Team
 */

add_action( 'init', array( 'XTeam_Nav_Menu_Item_Custom_Fields', 'setup' ) );

class XTeam_Nav_Menu_Item_Custom_Fields {
	static $options = array(
		'item_tpl' => '
			<p class="additional-menu-field-{name} description description-thin">
				<label for="edit-menu-item-{name}-{id}">
					{label}<br>
					<input
						type="{input_type}"
						id="edit-menu-item-{name}-{id}"
						class="widefat code edit-menu-item-{name}"
						name="menu-item-{name}[{id}]"
						value="{value}">
				</label>
			</p>
		',
	);

	static function setup() {
		if ( !is_admin() )
			return;

		$new_fields = apply_filters( 'xteam_nav_menu_item_additional_fields', array() );
		if ( empty($new_fields) )
			return;
		self::$options['fields'] = self::get_fields_schema( $new_fields );

		add_filter( 'wp_edit_nav_menu_walker', function () {
			return 'XTeam_Walker_Nav_Menu_Edit';
		});
		//add_filter( 'xteam_nav_menu_item_additional_fields', array( __CLASS__, '_add_fields' ), 10, 5 );
		add_action( 'save_post', array( __CLASS__, '_save_post' ), 10, 2 );
	}

	static function get_fields_schema( $new_fields ) {
		$schema = array();
		foreach( $new_fields as $name => $field) {
			if (empty($field['name'])) {
				$field['name'] = $name;
			}
			$schema[] = $field;
		}
		return $schema;
	}

	static function get_menu_item_postmeta_key($name) {
		return '_menu_item_' . $name;
	}

	/**
	 * Inject the 
	 * @hook {action} save_post
	 */
	static function get_field( $item, $depth, $args ) {
		$new_fields = '';
		foreach( self::$options['fields'] as $field ) {
			$field['value'] = get_post_meta($item->ID, self::get_menu_item_postmeta_key($field['name']), true);
			$field['id'] = $item->ID;
			$new_fields .= str_replace(
				array_map(function($key){ return '{' . $key . '}'; }, array_keys($field)),
				array_values(array_map('esc_attr', $field)),
				self::$options['item_tpl']
			);
		}
		return $new_fields;
	}

	/**
	 * Save the newly submitted fields
	 * @hook {action} save_post
	 */
	static function _save_post($post_id, $post) {
		if ( $post->post_type !== 'nav_menu_item' ) {
			return $post_id; // prevent weird things from happening
		}

		foreach( self::$options['fields'] as $field_schema ) {
			$form_field_name = 'menu-item-' . $field_schema['name'];
			// @todo FALSE should always be used as the default $value, otherwise we wouldn't be able to clear checkboxes
			if (isset($_POST[$form_field_name][$post_id])) {
				$key = self::get_menu_item_postmeta_key($field_schema['name']);
				$value = stripslashes($_POST[$form_field_name][$post_id]);
				update_post_meta($post_id, $key, $value);
			}
		}
	}

}

// @todo This class needs to be in it's own file so we can include id J.I.T.
// requiring the nav-menu.php file on every page load is not so wise
require_once ABSPATH . 'wp-admin/includes/nav-menu.php';
class XTeam_Walker_Nav_Menu_Edit extends Walker_Nav_Menu_Edit {
	function start_el(&$output, $item, $depth, $args) {
		$item_output = '';
		parent::start_el($item_output, $item, $depth, $args);
		// Inject $new_fields before: <div class="menu-item-actions description-wide submitbox">
		if ( $new_fields = XTeam_Nav_Menu_Item_Custom_Fields::get_field( $item, $depth, $args ) ) {
			$item_output = preg_replace('/(?=<div[^>]+class="[^"]*submitbox)/', $new_fields, $item_output);
		}
		$output .= $item_output;
	}
}


// Somewhere in config...
add_filter( 'xteam_nav_menu_item_additional_fields', 'mytheme_menu_item_additional_fields' );
function mytheme_menu_item_additional_fields( $fields ) {
	$fields['color'] = array(
		'name' => 'color',
		'label' => __('Color', 'xteam'),
		'container_class' => 'link-color',
		'input_type' => 'color',
	);
	
	return $fields;
}

@kucrut great use of filters for providing the additional fields.

Great Tutorial ....

How do i get the custom filed value?

Is this right way?

$a = get_post_meta($item->object_id,'custom_field',true);

or any other ..?

Hmm. This didn't work for me either. Is there any other sources online about adding new input fields to the wordpress menu editor?

how to get additional fileds value bro ?

For anyone else coming here wondering how to get the value:

You need to prepend your field name with "_menu_item". So if you have a field name "anchor":

get_post_meta($item->ID, '_menu_item_anchor', true)

Would you be able to help me figure out how to make this work for a checkbox? Also a trimmed down version for a single field would be awesome (but I can probably figure that out on my own).

For anyone who wonders how to use on the frontend:

add_filter( 'nav_menu_link_attributes', 'my_nav_menu_attribs', 10, 3 );

function my_nav_menu_attribs( $atts, $item, $args ) {
  $atts['data-mydata'] = get_post_meta($item->ID, '_menu_item_mydata', true);
  return $atts;
}

@kucrut, thanks for sharing!

Does anyone know how to display the additional fields in the menu editor inside Customizer?

@JacobDB: Did you get the checkbox to work? I've managed to convert the fields, but haven't yet been succesful in saving (and retrieving) them to/from the database.

If anyone here - how to add checkbox and/or select input ?

if anyone gets to here, this is now packaged into a library that you can use

How to do when there's two or more fields 'item_tpl', 'item_tpl2', 'item_tpl3'

I got an error when I do this code below.

$new_fields .= str_replace(
array_map(function($key){ return '{' . $key . '}'; }, array_keys($field)),
array_values(array_map('esc_attr', $field)),
self::$options['item_tpl', 'item_tpl2', 'item_tpl3']
);

Please help Thanks!

Hi friends,how to add checkbox and/or select input ?
and how to get values ??