How to create a custom WordPress Rest Api route. https://since1979.dev/snippet-005-simple-custom-rest-api-route/

snippet-004-rest-api-callback.php

<?php
/**
 * mytheme_handle_get_meta_value.
 *
 * Handle calls to the Wp Rest Api /meta end-point,
 *
 * @see https://since1979.dev/snippet-005-simple-custom-rest-api-route/
 *
 * @uses get_post_meta() https://developer.wordpress.org/reference/functions/get_post_meta/
 * @uses WP_Error() https://developer.wordpress.org/reference/classes/wp_error/
 * @uses rest_ensure_response() https://developer.wordpress.org/reference/functions/rest_ensure_response/
 * @uses array() https://www.php.net/manual/en/function.array.php
 */
function mytheme_handle_get_meta_value($request)
{
    $post = $request['post'];
    $key = $request['key'];
    $value = get_post_meta($post, $key, true);

    if (!$value)
        return new WP_Error('Meta value not found', 'Invalid meta key', array('status' => 404));

    return rest_ensure_response(array('meta' => $value));
}
?>