Thursday, October 09, 2008

Suppressing and Logging JavaScript Error messages

When it comes time to run your fancy AJAX site on production, you may find that there will occasionally be minor JavaScript errors (possibly from 3rd party sites, never your own of course) which will not only prevent your scripts from running but may also pop up some warning boxes on the end users' systems (especially if your target audience is made up of web developers who have "Disable JavaScript Debugging" unchecked).

Even if the user does not have debugging turned on, they may still see the error symbol in the status bar (and this does not exactly put their fears at ease, especially if they are trying to make a cash transaction).

Fortunately, you can suppress these error messages by using the following JavaScript code

function noError(){return true;}
window.onerror = noError;

Essentially what this does is catch any JavaScript errors and passes them to a null function. You should probably wrap this in a conditional which will only run on production and staging because as you are developing you will want to see these errors to prevent them from happening in the first place (i.e.
<%if ENV['RAILS_ENV'] == 'production' || ENV['RAILS_ENV'] == 'staging' %>
<script type="text/javascript">
function noError(){return true;}
window.onerror = noError;

<% end %>

window.onerror, as it happens, can also pass in some useful variables if you want to log them somehow. These are the error message, the url and the linenumber.

function alertError(message, url, linenumber){
alert(message + " " + url + " " + linenumber)
return true;
window.onerror = alertError;

However, what you probably really want to do is log it to your server via AJAX as opposed to alerting the error message to the user (using jQuery below).

function logError(message, url, linenumber){
type: 'GET',
url: '/javascript_error',
data: {message:message, url:url, linenumber:linenumber},
dataType: 'json'
return true;
window.onerror = logError;

On the server side, create a small function which will take in these parameters and log them along with the user's remote address and user agent ( request.remote_addr and request.user_agent in Rails).

There are some browser differences (and inconsistencies) with this however. For a start, while Firefox will give you the url of the JavaScript file in which the error occurs, Internet Explorer will only give you the url of the page that is rendered in the browser (i.e. what is in the location box). Safari does not seem to support these at all (Safari will suppress the error, but not run the alertError function) and IE in debug mode behaves the same as Safari.


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Dumitru Glavan said...

It's a bit more complex to log the clientside errors, supporting all the major browsers and mobile devices. We built to take care about that. It's also possible to log the backend errors with the NodeJS library. Worth to give it a try.