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

[Tushar8649]

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🔹 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

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🔹 Types of Client Scripts

ServiceNow mainly provides 4 types:

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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

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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

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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

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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

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🔹 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);

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🔹 Execution Flow

Simple way to understand:

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

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🔹 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

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🔹 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

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🔹 Common Mistakes

• Using GlideRecord in client script

• Missing isLoading check

• Too many alerts

• Writing large logic in onChange

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🔹 Real Use Cases

• Auto set Impact and Urgency

• Validate mandatory fields

• Auto assign group

• Show/hide fields based on conditions

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🔹 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);
});

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🔹 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.

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

[saraw925]

Nice. Thank you for sharing.