Client Script Question...

Hi I have written this bit client script but it doesnt seem to be playing ball, essentially I want a notification to ping, when a user goes to edit the field "Total_Refund" but if it exceeds "Total_Spend" it clears the value and notifies accordingly. Can anyone help?

{
    // Get references to the necessary fields
    totalSpendField = input.Total_Spend;
    refundField = input.Total_Refund;

    // Attach an event listener to the refund field for dynamic validation
    refundField.on("change", validateRefund);
}

// Validation function
validateRefund()
{
    totalSpend = totalSpendField.get(); 
    refundValue = refundField.get(); 

    if (refundValue > totalSpend)
    {
        alert("Refund cannot be greater than the total spend (£" + totalSpend + ").");
    refundField.set(""); // Clear the invalid refund value
    }
}