zewpgists_reorder_posts_by_title.php

<?php
/* 
 * Order posts by title 
 * 
 * @see https://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts 
 *  
 * Notes:  
 * - 'zewpgists_' is a prefix for all my gists. Replace with your own. 
 * 
 */

add_action( 'pre_get_posts', 'zewpgists_order_posts_by_title' );

/** 
 * Order the main query by title 
 * 
 * @param  object $query Main query. 
 * 
 * @return object        Reordered query. 
 */
function zewpgists_order_posts_by_title( $query ) {	

	/*	 
	 * Apply only to the main query and on the main posts loop	 
	 *	 
	 * @see https://codex.wordpress.org/Function_Reference/is_main_query	 
	 * @see https://codex.wordpress.org/Function_Reference/is_front_page	 
	 */ 	
	if ( $query->is_main_query() && $query->is_front_page() ) {		

		$query->set( 'orderby', 'title' ); // Default is post_date.		
		$query->set( 'order', 'ASC' ); // Default is DESC.	

	}
	
}