ServiceNow JavaScript: Server Side vs Client Side

This document consolidates all major scripting types and JS usage on the Now Platform into one reference. Use it as your active note-taking and recall guide.

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Universal JS on Now Platform Template

Feature:

Purpose:
- Why this script exists
- What problem it solves

Runs:
- Client | Server | Both
- Trigger / Event / Flow

APIs / Globals:
- List of platform APIs (g_form, GlideRecord, GlideAjax, GlideFlow, etc.)

Inputs:
- Fields, variables, Flow inputs, or app variables

Outputs:
- Updated fields, records, logs, Flow outputs

Use Cases:
- Concrete scenarios where this should be used
- Where NOT to use it

Common Mistakes / Gotchas:
- Misfires, infinite loops, security issues, context mistakes

1-Min Example:
- Step-by-step scenario with input → JS → output

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Client-Side Scripting

Client Script

 Client Script (onChange example)

Purpose:
- Control form field behavior

Runs:
- Client, triggered by field change

APIs / Globals:
- g_form, g_user

Inputs:
- Field value changed

Outputs:
- Field visibility / mandatory / value set

Use Cases:
- Show/hide fields dynamically
- Mandatory logic on form
- NOT for DB changes

Common Mistakes / Gotchas:
- Trying to modify database fields directly
- Forgetting runs only in the browser

1-Min Example:
- If “Category” = Hardware → make “Serial Number” mandatory

UI Policy

 UI Policy

Purpose:
- Declarative form behavior (low/no script)

Runs:
- Client, on form load/change

APIs / Globals:
- UI Policy Actions

Inputs:
- Field values

Outputs:
- Show/hide fields, mandatory logic

Use Cases:
- Simple form logic without scripts
- NOT for complex validations

Common Mistakes / Gotchas:
- Using script when policy can do it
- Only client-side execution

1-Min Example:
- If category = Hardware → show serial number

UI Action

 UI Action

Purpose:
- Add buttons / actions to UI

Runs:
- Client, Server, or Both, on click

APIs / Globals:
- g_form, current

Inputs:
- Button clicked

Outputs:
- Updates fields, triggers logic

Use Cases:
- User-triggered actions
- NOT for background enforcement

Common Mistakes / Gotchas:
- Mixing client/server logic incorrectly
- No condition guarding visibility

1-Min Example:
- "Escalate Incident" button

UI Page

 UI Page

Purpose:
- Custom UI with HTML/JS

Runs:
- Client + Server (Jelly)

APIs / Globals:
- GlideAjax, DOM

Inputs:
- Form data, user input

Outputs:
- Custom display, server updates

Use Cases:
- Custom dashboards or forms
- NOT for simple forms

Common Mistakes / Gotchas:
- Reinventing OOB UI
- Security issues

1-Min Example:
- Custom dashboard input form

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Server-Side Scripting

Business Rule

 Business Rule

Purpose:
- Enforce data rules at save time

Runs:
- Server, Before / After / Async

APIs / Globals:
- current, previous, GlideRecord, gs

Inputs:
- Record fields

Outputs:
- Updated record, notifications, logs

Use Cases:
- Data integrity enforcement
- Audit logs
- NOT for UI behavior

Common Mistakes / Gotchas:
- Doing UI logic
- Wrong timing (Before vs After)
- Infinite loops

1-Min Example:
- If incident.state = Closed → set closed_at timestamp

Script Include

Script Include

Purpose:
- Reusable server-side logic

Runs:
- Server, callable by other scripts

APIs / Globals:
- GlideRecord, gs

Inputs:
- Varies by use

Outputs:
- Return value, updated records

Use Cases:
- Reusable logic across BRs, Flows, APIs
- NOT for UI logic

Common Mistakes / Gotchas:
- Putting logic directly in BRs
- Forgetting client-callable flag

1-Min Example:
- Calculate SLA based on priority

Scheduled Script

Scheduled Script

Purpose:
- Run logic on a schedule

Runs:
- Server, time-based (cron)

APIs / Globals:
- GlideRecord, gs

Inputs:
- Scheduled trigger

Outputs:
- Updated records, logs

Use Cases:
- Cleanup jobs, batch updates
- NOT for real-time logic

Common Mistakes / Gotchas:
- Heavy processing during business hours
- Missing logging

1-Min Example:
- Nightly close stale incidents

Fix Script

Fix Script

Purpose:
- One-time data correction

Runs:
- Server, manual execution

APIs / Globals:
- GlideRecord

Inputs:
- Target records

Outputs:
- Updated records

Use Cases:
- Backfilling data, correcting bad records
- NEVER for ongoing logic

Common Mistakes / Gotchas:
- Running in prod without test
- Forgetting rollback plan

1-Min Example:
- Set missing assignment groups

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Flow / Automation

Flow Script Step

Flow Script Step

Purpose:
- Custom logic inside Flow Designer

Runs:
- Server, triggered by Flow

APIs / Globals:
- GlideRecord, Flow inputs/outputs

Inputs:
- Flow variables

Outputs:
- Updated Flow variables, records

Use Cases:
- Conditional logic not supported declaratively
- NOT for UI logic

Common Mistakes / Gotchas:
- Overly complex logic inside Flow
- Not returning outputs correctly

1-Min Example:
- If "priority" > 3 → assign to escalation group

App Engine / Scoped App Script

Scoped App Script (Server-side)

Purpose:
- Encapsulate app-specific logic

Runs:
- Server, callable from other modules

APIs / Globals:
- GlideRecord, gs, Script Includes

Inputs:
- App variables, table fields

Outputs:
- Updated records, app variables

Use Cases:
- Reusable app logic, integrations
- NOT for unrelated tables

Common Mistakes / Gotchas:
- Forgetting scope protection
- Using global objects incorrectly

1-Min Example:
- Calculate SLA for custom app records

Report Script

Report Script

Purpose:
- Customize report logic / data retrieval

Runs:
- Server during report generation

APIs / Globals:
- GlideRecord, gs

Inputs:
- Table data, filters

Outputs:
- Report dataset

Use Cases:
- Advanced filtering, calculated fields
- NOT for UI actions

Common Mistakes / Gotchas:
- Slow queries
- Not respecting permissions

1-Min Example:
- Calculate overdue incidents grouped by assignment group

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Server vs Client Method Tips

Quick Guide to Remember

• g_form → Client only (form manipulation, field visibility, mandatory fields)
• g_user → Client only (user info in browser)
• current → Server only (record being saved or queried)
• previous → Server only (record state before update)
• GlideRecord → Server only (query/update database)
• gs → Server only (logs, notifications, user info)
• GlideAjax → Client-to-server bridge (client calls server methods)
• Flow APIs → Server (Flow Script Step)
• UI Policy Actions → Client (show/hide, mandatory, read-only)

Mental Shortcut

• Touching the database = server
• Touching the form/UI = client
• Reusable logic = Script Include / Scoped App
• Scheduled/Timed = Scheduled Script
• One-time fix = Fix Script

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Key JS Patterns to Memorize

• If it touches the DB → Server
• If it touches the form → Client
• If it’s reusable logic → Script Include / Scoped App
• If it’s a process / workflow → Flow Script Step
• Always check API scope: g_form = client, current = server, GlideAjax = client→server bridge

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This document is designed for active note-taking and hands-on reinforcement. Fill in the 1-Min Example and Use Cases sections with examples from your dev instance to lock in memory.