Summary
Most ServiceNow system administrators have had some experience working with client-side JavaScript. It is often very obvious and familiar to write scripts that interact with the web browser. Validating information using the browser will mean that your users can generally work more quickly with the application rather than clicking and waiting.
However, client-side scripts do have significant drawbacks. They should not be used to guarantee data integrity and enforce security controls since a malicious user can easily manipulate the browser to disable the checks. Client-side scripts can easily start to slow the browser down in situations such as when there are dozens of fields that are being hidden by a UI Policy.
However, there are many other ways of providing a great user experience. UI Actions are a great way to carry out complex actions in a single click. They can run code on the server, or in the browser, and can be placed on a form or in a list. Use them to simplify the interface...
