ServiceNow Client scripts: A Practical Guide

ServiceNow Client scripts

Run JavaScript in the browser to react to form and field events in real time.

Quick answer

ServiceNow client scripts run JavaScript in the browser on a form, the counterpart of server-side business rules. Four types: onLoad when the form opens, onChange when one field changes, onSubmit as last-chance validation that can abort, onCellEdit for inline list edits. The g_form API sets values, mandatory and visibility; guard onChange with isLoading and keep logic on the server.

Key takeaways

The four types
An onChange script
Keep them light
Try it Yourself

Client scripts run in the user's browser on a form, letting you validate input, show or hide fields, and react the instant a value changes, without a round-trip to the server. They are the counterpart to server-side business rules: same language (JavaScript), opposite side of the wire.

The four types

onLoad

Runs once when the form loads, set defaults, hide fields.

onChange

Runs when a specific field changes, the workhorse.

onSubmit

Runs when the form is saved, last-chance validation, can abort.

onCellEdit

Runs on inline list edits.

An onChange script

// Make "Justification" mandatory when priority is Critical
function onChange(control, oldValue, newValue, isLoading) {
  if (isLoading || newValue === '') return;
  g_form.setMandatory('justification', newValue == '1');
}

The g_form API is your toolbox: g_form.getValue(), setValue(), setMandatory(), setVisible(), setReadOnly(), addErrorMessage(). Use g_user to check roles on the client.

Do not query the database from the client. Client scripts have no direct database access. When you need server data, call a GlideAjax Script Include asynchronously, never synchronous getReference or GlideRecord on the client, which freezes the form.

Keep them light

Prefer UI policies for simple show/hide/mandatory, no code, faster.
Always guard onChange with the isLoading check.
Move real logic to the server; the client should only shape the form.

Try it YourselfJavaScript

▸ Press Run to execute.

Runs in a sandbox in your browser. Mock gs and GlideRecord and sample data are provided.