Development · Lesson

Client scripts

Quick answer

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.

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.

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Check your understanding

Client scripts run...
- A. In the browser
- B. On the database server
- C. On the MID Server
Show answer
A. In the browser
Client scripts execute in the user's browser.
Which type reacts to a single field changing?
- A. onChange
- B. onLoad
- C. onSubmit
Show answer
A. onChange
onChange fires when its configured field changes.
To fetch server data from a client script you should use...
- A. GlideAjax (async)
- B. Synchronous GlideRecord
- C. Nothing, it can't
Show answer
A. GlideAjax (async)
GlideAjax retrieves server data asynchronously without freezing the form.

Frequently asked questions

What does the term Client scripts refer to in ServiceNow?

What is the practical takeaway on Client scripts?

Prefer UI policies for simple show/hide/mandatory, no code, faster. Always guard onChange with the isLoading check.