MAINTAINABILITY
In early websites, JavaScript was used primarily for small effects or form validation. Today's web applications are filled with thousands of lines of JavaScript executing all types of complicated processes. This evolution requires that developers take maintainability into account. As with software engineers in more traditional disciplines, JavaScript developers are hired to create value for their company, and they do that not just by delivering products on time but also by developing intellectual property that continues to add value long after.
Writing maintainable code is important because most developers spend a large amount of their time maintaining other people's code. It's a truly rare occurrence to be able to develop new code from scratch; it's often the case that you must build on work that someone else has done. Making sure that your code is maintainable ensures that other developers can perform their jobs as well as possible.
