The error console is a utility built in to your browser that shows error messages whenever a script error is encountered.
Constant use of the error console is essential in tracking-down errors in scripts, particularly during development.
Failure to use the error console during development/debugging will cause unnecessary frustration and lost time.
At the time of writing, the browser with the most useful built-in error console is Mozilla FireFox. It is accessed via the Tools menu. In addition there is a Firefox debugger plugin available called Firebug .