A.1. Global Variables

The worst of all of JavaScript's bad features is its dependence on global variables. A global variable is a variable that is visible in every scope. Global variables can be a convenience in very small programs, but they quickly become unwieldy as programs get larger. Because a global variable can be changed by any part of the program at any time, they can significantly complicate the behavior of the program. Use of global variables degrades the reliability of the programs that use them.

Global variables make it harder to run independent subprograms in the same program. If the subprograms happen to have global variables that share the same names, then they will interfere with each other and likely fail, usually in difficult to diagnose ways.