A List Apart: Articles: JavaScript Logging:

"Another, simpler debugging method has come into fashion in recent years, although the principle behind it must be as old as the art of programming itself. This method is called logging, and it works by echoing as much information to the screen as is necessary to find out how your code and data structures are behaving. The echoed information can include everything from descriptive statements that are used to let the programmer know what is happening in the script to the value of a variable or the field of an object; it can be any information that will inform the author of what is happening in the code."