On Change Client Script In ServiceNow

OnChange():

    Runs when a field value changes.

1. Control (Field Name)

• This tells the script which field on the form just changed.
• It is the name of the object whose value was updated.
• Example — if the "Assigned To" field changed, then Control = assigned_to.
• You define this field name inside the Client Script form itself.

2. OldValue (Value Before Change)

• This is the value the field had when the form first loaded.
• It stays the same no matter how many times the field changes after loading.
• Example — if Assigned To changed from Jai → Abhijeet, then OldValue = Jai.
• Even if you later change it again to someone else, OldValue will always remain Jai.

3. NewValue (Value After Change)

• This is the latest value of the field after the change happened.
• It updates every time the user makes a new selection or input.
• Example — if Assigned To changed from Jai → Abhijeet, then NewValue = Abhijeet.
• Your script can use this to trigger actions based on the new value.

4. IsLoading (Is the Form Still Loading?)

• This is a True / False value.
• True = the field changed because the form is still loading — not because the user did something.
• False = the user manually changed the field after the form fully loaded.
• Very useful to avoid running unnecessary actions during form load.
• Example — you don't want an alert to pop up every time the form opens, so you check IsLoading = false first.

5. IsTemplate (Did a Template Fill This Field?)

• This is also a True / False value
• True = the field was filled automatically by a pre-defined template, not the user
• False = the user changed the field manually themselves
• Helps your script understand why the value changed — user action or template action
• Example — if a template auto-fills the "Priority" field, IsTemplate = true

Parameters:

    function onChange(control, oldValue, newValue, isLoading, isTemplate)

Use Cases: 

    ● Auto-fill fields 

    ● Validate field values 

    ● Trigger logic based on selection 

1. Set Value to Priority 1.

ServiceNow Code

function onChange(control, oldValue, newValue, isLoading) {
  if (isLoading || newValue == '') {
    return;
  }
  if (newValue == '1') {
    g_form.setValue('priority', '1');
  }
}

2. Make Field Mandatory Based on Priority .

ServiceNow Code

function onChange(control, oldValue, newValue, isLoading) {
  if (isLoading || newValue == '') {
    return;
  }
  if(newValue == '1'){ 
        g_form.setMandatory('description', true); 
    } 
}

3. Hide Field Based on Category.

ServiceNow Code

function onChange(control, oldValue, newValue, isLoading) {
  if (isLoading || newValue == '') {
    return;
  }
  if(newValue == 'software'){ 
        g_form.setDisplay('subcategory', false); 
    }
}

4.  Validate Field Length.

ServiceNow Code

function onChange(control, oldValue, newValue, isLoading) {
  if (isLoading || newValue == '') {
    return;
  }
  if(newValue.length < 10){ 
        g_form.showFieldMsg('description','Minimum 10 characters required','error'); 
    } 
}

5. Auto Set Assignment Group.

ServiceNow Code

function onChange(control, oldValue, newValue, isLoading) {
  if (isLoading || newValue == '') {
    return;
  }
  if(newValue == 'network'){ 
        g_form.setValue('assignment_group','Network Team'); 
    } 
}

6. Make field read only when state = Resolved.

ServiceNow Code

function onChange(control, oldValue, newValue, isLoading) {

  // Stop script running on form load
  if (isLoading) return;

  // Get current state value
  var state = g_form.getValue('state');

  // Lock fields when incident is resolved
  if (state === '6') {  // 6 = Resolved
    g_form.setReadOnly('short_description', true);
    g_form.setReadOnly('description', true);
    g_form.setReadOnly('category', true);
    g_form.addErrorMessage('Incident is Resolved — fields are locked');

  } else {
    g_form.setReadOnly('short_description', false);
    g_form.setReadOnly('description', false);
    g_form.setReadOnly('category', false);
    g_form.clearMessages();
  }
}

7. Auto-populate field based on Priority value.

ServiceNow Code

function onChange(control, oldValue, newValue, isLoading) {

  // Stop script running on form load
  if (isLoading) return;

  // Get the priority value user just selected
  var priority = g_form.getValue('priority');

  // Auto-set Impact based on Priority
  if (priority === '1') {
    g_form.setValue('impact', '1');      // High impact
    g_form.setValue('urgency', '1');     // High urgency
    g_form.addInfoMessage('P1 Incident — Notify Manager immediately');

  } else if (priority === '2') {
    g_form.setValue('impact', '2');      // Medium impact
    g_form.setValue('urgency', '2');     // Medium urgency

  } else {
    g_form.clearMessages();             // Clear banner messages
  }
}