Jan 15, 2016

jQuery.get() Ajax call fails on localhost url

While going through the book Learning jQuery, 4th Edition, I've been doing the exercises by placing and editing code files in a local directory on the desktop and opening index.html as a file in Firefox.

This worked fine until I had to code an Ajax call that referred to a php file. This required the use of a web server, so I created a subdirectory under the local server root and copied the php file into the subdir.

Keeping all other code on the desktop, I then edited the Ajax call so that it requested from the url


The call silently failed and returned nothing. Console windows would show nothing, no errors.

Eventually, I found out about the same-origin policy. In brief, this policy means that by default JavaScript is prevented from making requests across domain boundaries, where same-origin means that the protocol, hostname, and port number must be identical.

So I moved all code files into the subdir under the server root. Working from there rather than the desktop, the Ajax calls then succeeded.

(And I changed the requested url back to just e.php)


But what about carrying out cross-origin access? One possible way is to configure the server to allow it. For Apache on my Linux system, I added the following directive in /etc/apache2/apache2.conf

  <Directory /var/www/html/learning-jquery>
      Header set Access-Control-Allow-Origin "*"

However, checking this configuration change with the command

  # apachectl -t

resulted in

AH00526: Syntax error on line 205 of /etc/apache2/apache2.conf:
Invalid command 'Header', perhaps misspelled or defined by a module not included in the server configuration
Action '-t' failed.
The Apache error log may have more information.

It turns out that I also had to enable a module named headers

  # a2enmod  headers

Then restart the server. That enabled cross-origin access.

Not yet satisfied, I was wondering if the wildcard "*" in the directive could be replaced by something more restrictive. For the way I'm developing locally, the following also works because opening a file via the file:// protocol results in an origin of null.

  <Directory /var/www/html/learning-jquery>
      Header set Access-Control-Allow-Origin "null"

The fact that the calls had failed silently was very troubling. One way to avoid this is to register a global Ajax error handler by calling ajaxError(). Here's a handler that simply puts up an alert box. (I'm using jQuery 1.9)

  function setAjaxErrorHandler() {
      function alertError(event, jqxhr, settings, thrownErr) {
        alert('Ajax error handler: ' + jqxhr.status 

                + '  ' + jqxhr.statusText);

For the cross-origin error, this puts up the message "Ajax error handler: 0  error", which is not exactly super helpful but better than nothing.

Another way is to attach an error handler to the particular request by using the fail() method. For example,

$.get('http://localhost/learning-jquery/e.php', { ... }, 
       function (data) { ... })
    .fail(function (jqxhr) {
        alert('GET error: ' + jqxhr.status + '  ' 

                + jqxhr.statusText);



Same-origin policy

Why is CORS important?

CORS on Apache

Configure Apache To Accept Cross-Site XMLHttpRequests on Ubuntu


  1. Super helpful! Perhaps the error handling could be added to the module?

  2. That's a good idea to look into adding error handling to the JS code for the Optimizely module.