[WordPress] Specifying custom error codes when using Ajax in WordPress.

00-acme-retrieve-information.php

<?php

add_action( 'wp_ajax_retrieve_information', 'acme_retrieve_information' );
add_action( 'wp_ajax_nopriv_retrieve_information', 'acme_retrieve_information' );
/**
 * Retrieves information for the user based on the specified post ID,
 * the user ID, and whether or not the user has currently voted on this
 * particular post.
 *
 * If information is not properly provided via the client-side, then one
 * of the follow error messages will be sent back to the browser:
 *
 * - '-1' means that the post ID was not specified
 * - '-2' means that the user ID was not specified
 * - '-3' means that the user has already voted
 *
 * Otherwise, the information will be retrieved and the value of said
 * information will be sent to the JavaScript.
 *
 * @since    1.0.0
 */
function acme_retrieve_information() {
	
	if ( ! isset( $_GET['post_id'] ) ) {
		
		$error = new WP_Error( '-1', 'The post ID was not set' );
      		wp_send_json_error( $error );
		
	}
	
	if ( ! isset( $_GET['user_id'] ) ) {
		
		$error = new WP_Error( '-2', 'The user ID was not set' );
      		wp_send_json_error( $error );
		
	}
	
	if ( ! acme_get_user_has_voted( $post_id, $user_id ) ) {
		
		$error = new WP_Error( '-3', 'The user has not voted' );
      		wp_send_json_error( $error );
		
	}
	
	/* If we reach this point, then we have everything we need in order
	 * to get the information the user is requesting. We can invoke the
	 * necessary function, store it in a variable, then echo it back to
	 * the user.
	 *
	 * Note this particular function's implementation is left out as it's
	 * irrelevant to the point being made in the code. 
	 *
	 */
	$information = acme_get_user_information( $post_id, $user_id );
  	if ( ! $information ) {
  		
      		$error = new WP_Error( '001', 'No user information was retrieved.', $information );
      		wp_send_json_error( $information );
  	}

  	wp_send_json_success( $information );
	
}

01-acme-retrieve-information.js

;(function( $ ) {
	'use strict';

	$(function() {

    $.ajax({

      url:    wpa_demo.ajax_url,
      method: 'GET',
      data:   { action: 'retrieve_information' }

		}).done(function( response ) {

			if ( true === response.success ) {

				response_json = $.parseJSON( response.data );
				// Handle the response.data here

			} else {

        // Otherwise, handle the errors or the messages that were provided by the server, here.
        error_codes    = response.data.codes;
        error_messages = response.data.messages;

			}

		});

	});

})( jQuery );
---
$.get( ajaxurl, {
  
  action:    'acme_retrieve_information',
  post_id:   $( '#post_id.hidden' ).text(),
  user_id:   $( '#user_id.hidden' ).text()
  
}, function( response ) {

  switch ( response ) {
    
    case '-1':
      // The post ID wasn't specified. Maybe it was empty in the hidden element, wasn't rendered in the template, etc.
      break;
    
    case '-2':
      // The user ID wasn't specified. Maybe it was empty in the hidden element, wasn't rendered in the template, etc.
      break;
      
    case '-3':
      // The user has already voted, so perhaps display a message saying such.
      break;
      
    default:
      
      if ( response.success ) {
        
        // Use response.data here.
        
      } else {
       
        // Otherwise, handle the errors or the messages that were provided by the server, here.
        error_codes    = response.data.codes;
        error_messages = response.data.messages;
        
      }
      
      break;
    
  }
  
});