When writing AJAX applications with CodeIgniter, you usually want to post to a controller class by using Javascript functions. For example, if you are using jQuery for posting a form or loading back a page using an AJAX style, then you need to call the Controller class from your jQuery code.

The way I usually do that is the following way:

I usually create a javascript object called CI, and make it globally accessible. Usually I place that variable in my header file (view), so it is defined anytime a page is displayed.

<script type="text/javascript">
<!--
    var CI = {
      'base_url': '<?php echo base_url(); ?>',
      'another': '<?php echo '' ?>',
      'another2': '<?php echo '' ?>'
    };
-->
</script>

Then, anytime you need to use your base_url(), site_url() or whatever you need (originally accessible via PHP) in a Javascript functions, you can simple call your CI.base_url

This way is very accessible so you don’t need to embed PHP function calls between your Javascript pieces of code.

Moreover, if you have external .js (javascript files), then you can easily handle these PHP or CodeIgniter variables.

So, how to use your Javascript CI variable?

Imagine you are using jQuery for your AJAX calls. Here, instead of using the <?php echo base_url() ?> we are willing to use our new CI object, as follows:

$.ajax({
        url: CI.base_url + 'time/ajax_clock',
        async:true,
        global: true,
        ifModified: false,
        processData:true,
        success: function(data){
        },
        type: "POST"
      });

Obviously you can extend your Javascript CI object using any other variable you may require in your application.

Related posts:

1. How do I display result or error messages in CodeIgniter application
2. CodeIgniter and AJAX using jQuery