Run onChange logic only when field…

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Sourdough: ServiceNow Monitoring and Analytics

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Instance:sourdoughdev·Version:Yokohama

Instance StateONLINE

System StatusFully Operational

Session Timeout90 minutes

Logged-In Sessions2 (20 active)

Build Nameyokohama-12-18-2024_p1

IP Address10.159.128.43

Instance HealthHealth Score: 90%

🔥 5dSourdough (Chrome Plugin)Dark Mode

Execute logic only when a field value changes, skipping form load

Client Script

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
    // Skip on form load - isLoading is true until all fields are populated
    if (isLoading) {
        return;
    }

    // Skip if value hasn't actually changed
    if (oldValue == newValue) {
        return;
    }

    // Skip template loads (new records from templates)
    if (isTemplate) {
        return;
    }

    // Only execute when priority actually changes
    var priority = g_form.getValue('priority');
    
    if (priority == '1') {
        // Critical - make assignment group mandatory
        g_form.setMandatory('assignment_group', true);
        g_form.showFieldMsg('assignment_group', 'Assignment group required for P1 incidents', 'info');
        
        // Auto-populate if empty
        if (!g_form.getValue('assignment_group')) {
            g_form.setValue('assignment_group', 'e85d934e6fa831005be8883e6b3ee4b8'); // IT Support
        }
    } else {
        // Non-critical - remove mandatory requirement
        g_form.setMandatory('assignment_group', false);
        g_form.hideFieldMsg('assignment_group');
    }

    // Log the actual change for debugging
    console.log('Priority changed from ' + oldValue + ' to ' + newValue);
}

ServiceNow fires onChange events during form load as each field gets populated, not just on user interaction. Without proper guards, your logic executes dozens of times before the user even sees the form. The isLoading parameter is your lifeline — it stays true until all fields finish loading. Most developers forget the oldValue == newValue check, causing logic to run even when calculated fields refresh with the same value.

When to use this pattern

When you need to enforce business rules based on field changes (mandatory fields, visibility, calculations)
When making GlideAjax calls that should only fire on actual user changes, not form load
When populating dependent fields or clearing related data based on user selections
When showing contextual field messages or alerts that should only appear after user interaction

When NOT to use this pattern

Don't use for one-time form initialization logic — use an onLoad Client Script instead
Don't use for expensive operations on every keystroke — debounce with setTimeout() or use onSubmit
Don't use when you need server-side field access — put the logic in a Business Rule instead
Don't use for complex validation — use Data Policies or server-side validation in Business Rules

Key behaviors and gotchas

Use == not === for value comparison — ServiceNow sometimes passes empty strings vs null inconsistently
The isTemplate parameter is true when loading records from templates — skip this to avoid premature logic execution
Reference fields pass display values in oldValue and newValue — use g_form.getValue() for sys_ids
Choice fields fire onChange even when users select the same option — the value comparison catches this
Calculated fields and UI Policies can trigger onChange events when they refresh — guard against this with value comparison
Mobile apps may fire onChange events differently — always test on mobile if your users access forms there

⚠️

Never call g_form.setValue() inside onChange for the same field — you'll create an infinite loop. Use a flag variable or move the logic to a different event if you need to modify the field that triggered the change.

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Alternative Approaches

Debounced onChange for expensive operations

var changeTimeout;

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
    if (isLoading || oldValue == newValue || isTemplate) {
        return;
    }
    
    // Clear existing timeout
    if (changeTimeout) {
        clearTimeout(changeTimeout);
    }
    
    // Debounce expensive operations by 500ms
    changeTimeout = setTimeout(function() {
        performExpensiveOperation();
    }, 500);
}

function performExpensiveOperation() {
    // Your expensive GlideAjax call or complex logic here
}

Use when you need expensive operations but want to avoid excessive API calls during rapid user input.

Trade-offs: Adds complexity and delays user feedback, but prevents performance issues with rapid field changes.

API Methods Used

GlideForm.getValue()GlideForm.setValue()GlideForm.setMandatory()GlideForm.showFieldMsg()

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Run onChange logic only when the field actually changes