JavaScript: The Good Parts

Appendix A. Awful Parts

That will prove awful both in deed and word.

—William Shakespeare, Pericles, Prince of Tyre

In this appendix, I present the problematic features of JavaScript that are not easily avoided. You must be aware of these things and be prepared to cope.

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.

Lots of languages have global variables. For example, Java's public static members are global variables. The problem with JavaScript isn't just that it allows them, it requires them. JavaScript does not have a linker. All compilation units are loaded into a common global object.

There are three ways to define global variables. The first is to place a var statement outside of any function:

var foo = value;

The second is to add a property directly to the global object. The ...