Get excerpt by id in Wordpress.

get_excerpt_by_id.php

<?php
if (!function_exists(get_excerpt_by_id)) {
    /**
     * Returns excerpt by ID. WP_Post->post_excerpt if it presents, wp_trim_words on other case.
     * @param  mixed  $post  WP_Post object or post ID. Accepts WP_Post object or post ID (integer or string);
     * @return string        Excerpt
     */
    function get_excerpt_by_id($post) {
        $return_excerpt = function($post) {
            if ($post->post_excerpt == '')
                return wp_trim_words($post->post_content, apply_filters( 'excerpt_length', 55 ), apply_filters( 'excerpt_more', ' ' . '[&hellip;]' ));

            return $post->post_excerpt;
        };

        if (!is_object($post)) {
            $post = get_post($post);
        }

        return apply_filters('the_excerpt', $return_excerpt($post));
    }
}