There are very often two way to do things in ServiceNow - point and click, or code. It's a good idea to use the point and click mechanism - it is often highly optimized, portable, maintainable, and upgradable. This book will look at many of these options, and guide you towards using them when possible. But sometimes you need total control. To be a ServiceNow master, you need to write scripts.
ServiceNow has hundreds of places where you can write code. Aside from a very few exceptions, JavaScript is used in all instances. JavaScript is a very flexible and powerful language and is most commonly known for its inclusion in almost every web browser. This has made it almost mandatory for web development these days, with its simple syntax allowing many people to quickly add simple logic to web pages with minimum effort. Taking advantage of this familiarity, ServiceNow uses JavaScript both on the server and client sides.