What is a GlideAjax call, and why do you need it?

What This Question Really Tests

This is a top-tier beginner question because it tests understanding of the client-server boundary — the most important architectural concept in ServiceNow development.

Why You Need GlideAjax

Client Scripts run in the browser. They can manipulate the form with g_form, but they cannot query the database directly. GlideRecord is a server-side-only API. So when a Client Script needs data from the database, it must ask the server — and GlideAjax is the mechanism for that request.

Think of it as making an AJAX call to a backend API endpoint, except the 'endpoint' is a Script Include method.

Step-by-Step Example

// ============================================
// STEP 1: Create the Script Include (server-side)
// Name: UserInfoHelper
// Accessible from: All application scopes
// ============================================
var UserInfoHelper = Class.create();
UserInfoHelper.prototype = Object.extendsObject(
  AbstractAjaxProcessor, {  // MUST extend this for GlideAjax
  
  // This method will be callable from the client
  getUserDepartment: function() {
    var userId = this.getParameter('sysparm_user_id');
    var gr = new GlideRecord('sys_user');
    if (gr.get(userId)) {
      return gr.getDisplayValue('department');
    }
    return 'Unknown';
  },
  
  type: 'UserInfoHelper'
});

// ============================================
// STEP 2: Create the Client Script (client-side)
// Type: onChange
// Field: caller_id
// ============================================
function onChange(control, oldValue, newValue, isLoading) {
  if (isLoading || newValue === '') return;
  
  // Create the GlideAjax call
  var ga = new GlideAjax('UserInfoHelper');  // Script Include name
  ga.addParam('sysparm_name', 'getUserDepartment'); // Method to call
  ga.addParam('sysparm_user_id', newValue);  // Pass the user's sys_id
  
  // Make the call (asynchronous — doesn't freeze the form)
  ga.getXMLAnswer(function(answer) {
    // 'answer' contains the return value from the server
    g_form.setValue('u_department', answer);
  });
}

How It Works

• Client Script creates a GlideAjax object pointing to the Script Include name
• Passes the method name via sysparm_name parameter
• Passes any custom data via additional sysparm_ parameters
• Calls getXMLAnswer() with a callback function — this is asynchronous
• ServiceNow server receives the request, instantiates the Script Include, calls the method, and returns the result
• The callback function receives the result and updates the form

Important Rules

• The Script Include must extend AbstractAjaxProcessor to be callable via GlideAjax
• Use this.getParameter('sysparm_xxx') to read parameters on the server side
• Always use the asynchronous pattern (getXMLAnswer) — the synchronous version (getXMLWait) freezes the browser
• Return values are always strings — if you need objects, use JSON.stringify() and JSON.parse()