Trigger client-side validation in onSubmit

The onSubmit client script is your last line of defense before data hits the server — but only if you understand that returning false blocks the save completely. Most developers miss that you need to call g_form.clearMessages() first — without it, old validation messages persist and confuse users. The pattern works by accumulating all validation failures before returning a single boolean, ensuring users see every problem at once instead of playing whack-a-mole with errors.

When to use this

• Complex business logic validation that requires multiple field values or calculations
• Cross-field validation where one field's value determines requirements for another
• Conditional mandatory fields that standard field dictionary rules can't handle
• Data quality validation that needs immediate user feedback before server processing

When NOT to use this

• Simple mandatory field validation — use field dictionary mandatory settings instead
• Validation requiring server-side data lookups — use synchronous GlideAjax calls or Business Rules instead
• Security-critical validation — always validate server-side too, as client scripts can be bypassed
• Mobile app forms — onSubmit scripts don't execute in ServiceNow mobile apps

Key behaviors and gotchas

• Returning false blocks the save completely — the form stays in edit mode with your error messages displayed
• Use g_form.showFieldMsg() not alert() — field messages stay visible while users fix issues
• Scripts execute in UI Action order — if you have multiple onSubmit scripts, lower order values run first
• Any onSubmit script returning false blocks the save — remaining scripts in the chain won't execute
• Field messages persist until clearMessages() or successful form submission — always clear at the start
• Use getValue() for sys_ids and getDisplayValue() for reference field names — mixing them creates comparison bugs
• Checkbox fields return string 'true'/'false' not boolean — always use string comparison for checkbox validation