javascript

Chapter 14: Error Handling and Debugging
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There ’ s less of a chance that an error will occur in the process of logging the error. Most Ajax
communication is handled through functionality wrappers provided by JavaScript libraries. If
that library ’ s code fails, and you ’ re trying to use it to log the error, the message may never get
logged.
Whenever a try - catch statement is used, it ’ s likely that the error should be logged. Here is an example:
for (var i=0, len=mods.length; i < len; i++){
try {
mods[i].init();
} catch (ex){
logError(“nonfatal”, “Module init failed: “ + ex.message);
}
}
In this code, logError() is called when a module fails to initialize. The first argument is “ nonfatal ” ,
indicating the severity of the error, and the message provides contextual information plus the true
JavaScript error message. Error messages that are logged to the server should provide as much
contextual information as possible to help identify the exact cause of the error.
Debugging Techniques
Before JavaScript debuggers were readily available, developers had to use creative methods to debug
their code. This led to the placement of code specifically designed to output debugging information in one
or more ways. The most common debugging technique was to insert alerts throughout the code
in question, which was both tedious, because it required cleanup after the code was debugged, as well as
annoying if an alert was mistakenly left in code that was used in a production environment. Alerts are no
longer recommended for debugging purposes, because several other, more elegant solutions are available.
Logging Messages to a Console
IE 8, Firefox, Opera, Chrome, and Safari all have JavaScript consoles that can be used to view JavaScript
errors. All three also allow you to write directly to the console from code. For this to work in Firefox,
you ’ ll need to have Firebug installed ( www.getfirebug.com ), since it ’ s the Firebug console that is used
in Firefox. IE 8, Firefox, Chrome, and Safari allow you to write to the JavaScript console via the console
object, which has the following methods:
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error( message ) — Logs an error message to the console
info( message ) — Logs an informational message to the console
log( message ) — Logs a general message to the console
warn( message ) — Logs a warning message to the console
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