Complete Guide to Client Scripts in ServiceNow (Beginner → Advanced)

Tushar8649 · 2 hours ago

🔹 What is a Client Script?

A Client Script is JavaScript that runs on the browser (client side) and controls how the form behaves.

Why we use it:

Improve user experience
Validate data before submission
Automate field behavior
Reduce unnecessary server calls

🔹 Types of Client Scripts

ServiceNow mainly provides 4 types:

1. onLoad()

Runs when the form loads.

function onLoad() {
   g_form.setValue('priority', 2);
}

Use cases:

Set default values
Hide/show fields
Initialize form data

2. onChange()

Runs when a field value changes.

function onChange(control, oldValue, newValue, isLoading) {
   if (isLoading || newValue === '') return;

   if (newValue == '1') {
      g_form.setValue('impact', 1);
   }
}

Use cases:

Auto-populate fields
Apply dependent logic
Dynamic updates

3. onSubmit()

Runs when user submits the form.

function onSubmit() {
   if (!g_form.getValue('short_description')) {
      alert('Short description is required');
      return false;
   }
   return true;
}

Use cases:

Validation before submission
Prevent incorrect data

4. onCellEdit()

Used in list view (not form).

function onCellEdit(sysIDs, table, oldValues, newValue, callback) {
   if (!newValue) {
      alert('Value cannot be empty');
      return false;
   }
   callback(true);
}

Use cases:

Validate inline editing
Control list-level changes

🔹 g_form (Most Important API)

This is the main object used in client scripts.

Common methods:

getValue() → get field value
setValue() → set field value
setDisplay() → show/hide field
setMandatory() → make field mandatory
setReadOnly() → make field read-only
addInfoMessage() → show info message
addErrorMessage() → show error

Example:

g_form.setMandatory('short_description', true);
g_form.setReadOnly('priority', true);

🔹 Execution Flow

Simple way to understand:

Form loads → onLoad()
Field changes → onChange()
Form submit → onSubmit()
List edit → onCellEdit()

🔹 Client vs Server Scripts

Client Script Server Script

Runs on browser	Runs on server
Fast execution	Slightly slower
No DB access	Full DB access
UI logic	Data processing

🔹 Best Practices

Always use:

if (isLoading || newValue === '') return;

Keep logic simple and readable
Avoid heavy processing
Use GlideAjax for server calls
Prefer UI Policies for simple conditions

🔹 Common Mistakes

Using GlideRecord in client script
Missing isLoading check
Too many alerts
Writing large logic in onChange

🔹 Real Use Cases

Auto set Impact and Urgency
Validate mandatory fields
Auto assign group
Show/hide fields based on conditions

🔹 GlideAjax (Client → Server)

When you need server data:

var ga = new GlideAjax('MyScriptInclude');
ga.addParam('sysparm_name', 'getData');
ga.getXMLAnswer(function(response) {
   g_form.setValue('field', response);
});

🔹 Final Thought

Client Scripts are mainly about:

Better UI + Real-time validation + Smart forms

If your forms feel dynamic and responsive, good client scripting is usually behind it.

If anyone has real project scenarios or edge cases, feel free to share. Always interesting to see how others are solving problems.