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Client Scripts vs UI Policies

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In this lesson, you will learn:

  • How client scripts and UI policies are alike, and how they differ
  • Which one runs first on a form
  • How to choose the right one

1 Two tools for the same form

Client scripts and UI policies both run in the browser and both change forms. So which should you use? 🤔 The short answer: use a UI policy when you can, and a client script when you must.

Client script compared with UI policy. UI policy: no code needed, easy to read, great for mandatory, visible, and read only, and runs after client scripts. Client script: needs JavaScript, can set values, show messages, stop a save, and handle list edits. Below, the order on form load: first onLoad client scripts, then UI policies, so the UI policy wins if both change the same field.
Figure 1. UI policies run after client scripts, so they win on the same field.

2 Side by side

UI policy Client script
Code needed? No (scripts optional) Yes
Mandatory / visible / read only ✅ Best choice ✅ Possible
Set a field value Only with a script ✅
Stop a save ❌ ✅ onSubmit
Control list editing ❌ ✅ onCellEdit
Runs when After client scripts First

3 Which runs first?

When a form loads, onLoad client scripts run first, then UI policies. So if both change the same field, the UI policy wins.

⚠️ Heads up: A classic puzzle: “My client script makes a field mandatory, but it isn’t!” Check for a UI policy on the same field. It runs later and can undo your script.

4 How to choose

  • Mandatory, visible, or read only? UI policy.
  • Set or clear a value, or show a message? Client script (or a UI policy script).
  • Check before saving? onSubmit client script.
  • Control list edits? onCellEdit client script.
  • Must work for imports and integrations too? Neither alone. Add a data policy or business rule.

💡 Good to know: ServiceNow recommends UI policies over client scripts for field attributes. They’re easier to read, and they don’t need a developer to change them.

⭐ Remember: UI policy first choice for field behavior. Client script for logic. On load: client scripts, then UI policies.

5 Let’s try it: see who wins

  1. Create an onLoad client script on Incident: g_form.setMandatory('cmdb_ci', true);
  2. Your Network needs a CI UI policy (Lesson 33.1) sets Configuration item to not mandatory when Category isn’t Network (Reverse if false).
  3. Open an incident with Category Software. Is Configuration item mandatory? Why not?
  4. Deactivate the test client script when you’re done.

6 Quick recap

  • Both run in the browser and change forms
  • UI policies: best for mandatory, visible, and read only
  • Client scripts: for values, messages, save checks, and list edits
  • onLoad client scripts run before UI policies
  • If both touch one field, the UI policy wins

7 Check yourself

Think of your answer first. Then tap to check.

1 You need to hide a field when State is Closed. Client script or UI policy? Show answer

UI policy.

2 Which runs first on form load: onLoad client scripts or UI policies? Show answer

onLoad client scripts.

3 You need to stop a save if a date is in the past. Which tool? Show answer

An onSubmit client script.

Nice work! You finished this lesson. 🎉

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