Get the Excerpt by Post ID in WordPress

get-the-excerpt-by-post-id-in-wordpress.php

<?php

/**
 * Return the post excerpt, if one is set, else generate it using the
 * post content. If original text exceeds $num_of_words, the text is
 * trimmed and an ellipsis (…) is added to the end.
 *
 * @param  int|string|WP_Post $post_id   Post ID or object. Default is current post.
 * @param  int                $num_words Number of words. Default is 55.
 * @return string                        The generated excerpt.
 */
function km_get_the_excerpt( $post_id = null, $num_words = 55 ) {

	$post = $post_id ? get_post( $post_id ) : get_post( get_the_ID() );
	$text = get_the_excerpt( $post );

	if ( ! $text ) {
		$text = get_post_field( 'post_content', $post );
	}

	$generated_excerpt = wp_trim_words( $text, $num_words );

	return apply_filters( 'get_the_excerpt', $generated_excerpt, $post );

}