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Error Handling in JavaScript

Error Handling in JavaScript

Have you ever been to a site and gotten a pop-up box telling you about a JavaScript error on the page? It can be really annoying.

What's worse is that the person responsible for maintaining the site doesn't even know that the error occurred.

This article isn't about ColdFusion error handling. That was covered in 2000 and 2001 in CFDJ by Charlie Arehart's four-part series, "Toward Better Error Handling," as well as by others. Rather, this article will tell you some ways to trap JavaScript errors on the client. More important, it'll tell you how to communicate these errors to ColdFusion automatically so you can write the errors to a log file or e-mail the errors to yourself and find out about them right away. Indeed, the trick we use may be useful to you in ways beyond just error handling.

Error Prevention
The primary way to prevent JavaScript errors in your application, of course, is to catch them during development. To do this you'll have to know when they occur. Browsers may be set by default to hide them. To correct that you must turn on error display in your browser. This means that you see any JavaScript errors on all sites you visit, but to my mind it's a small price to pay to see them in your own site.

It's essential to know about JavaScript errors on your site, especially when debugging. Before I enabled this in my browsers, I wasted hours trying to figure out the cause of problems that became obvious when I enabled error display in my browser.

To do this in Internet Explorer, go to Tools -> Options -> Advanced and check "Display a notification about every script error". To see errors in Netscape, you must use the JavaScript console. To see the console in Netscape Navigator 4x, you must type "javascript:" into the location window. This will bring up the JavaScript console in your browser window. JavaScript errors will be written to the console as they occur. You can also use the console to test individual JavaScript statements. NN6 allows you to have the JavaScript console display in a separate window by going to Tasks -> Tools -> JavaScript Console.

Error Trapping with JavaScript
One of the most common ways to trap errors in troublesome ColdFusion code is to wrap a <cftry>/<cfcatch> block around it. If you're not using this method, you really should look into it. Part 4 of Charlie Arehart's series (June 2002) dealt with this topic.

JavaScript actually has try/catch statements of its own. As in CF, this is used to test for errors that you can anticipate. The simple example I use in Listing 1 only shows the error message (called "description" in IE and "message" in NN). You can catch information other than the message/description in that scope as well. However, the other variables also differ from browser to browser. Variables available in one browser aren't necessarily available in another. Table 1 (from JavaScript: The Complete Reference by Thomas Powell and Fritz Schneider [Osborne McGraw-Hill]) illustrates some of the variables that will be available in selected browsers.

The main problem with depending on try/catch for error handling in JavaScript is that it was only introduced in IE5 and NN6. This leaves out a good portion of your audience. Additionally, you have to write try/ catch statements around every bit of troubling code. While you can nest JavaScript try/catch statements just as you can ColdFusion ones, the nature of JavaScript means that encompassing all of your code in try/catch statements just isn't practical. Besides, we don't have the time to rewrite all of our existing code to use try/catch. There's got to be a better way.

Enter the "onerror" Property
This property of the window object has been available since NN3 and IE4. It's a great way to deal with JavaScript errors on a universal level (sort of like <cferror> - see Part 3 of Charlie Arehart's series [February 2001]). It can be used to deal with any JavaScript errors on the entire page, making it a very powerful error handler. Yet it rarely gets used.

The onerror property is used in the window object and in some cases it's available in other JavaScript objects as well (like the img object). I only cover the onerror property of the window object here.

The onerror property tells the browser to execute a function when a JavaScript error occurs anywhere on the page. If this function returns a value of true, this will also disable the browser's default error handling, including displaying the error message to the user for those who've opted to see them (as above).

Listing 2 is a simple example of the onerror property in use. All this does is display a series of alert boxes when an error occurs. It isn't terribly useful, but does make an effective demo. Note the three parameters used in defining the onerror event handler (function). The information used wasn't explicitly passed to the script, but rather was part of how the browser naturally passes information to the onerror event. Note also that I put it before any other HTML. I did this to demonstrate that you could include this script (or one like it) in your Application.cfm and it would still work. Naturally, you could put any other JavaScript statements in this script.

Bringing ColdFusion into the Picture
Certainly you've gotten information from the browser to ColdFusion before, but for this technique to work smoothly, we must do it in such a way that we don't distract from the user's experience. Notice that no other window is opened, and that our current page is not forced to refresh (which would likely cause the error to recur and put the user in an infinite loop). Any of that could interfere with the user's experience. So how do we do it?

The trick is getting images. Any time you use JavaScript to define an image source, you're making a trip to the server to get that image so it will be ready for use on the page. The source of an image object doesn't have to be an actual image. JavaScript doesn't check what sort of file you're using as the source. So you can call a ColdFusion page via the JavaScript image source functionality. You can even append variables to the URL in your request and JavaScript will dutifully send the request to the page and use that page as the source of an image.

This would cause a problem if you actually tried to display the image, but we have no need to do that. We're just sending information to a ColdFusion page. The image is simply our way to do it without forcing the user to leave the page. This is the only way I know of that this can be done in HTML, that is consistent across browsers, and that doesn't distract from the user's experience.

Once we know this, everything else is just an extension of what we've learned so far. The technique could have other uses as well. For example, you could pull a ColdFusion page that's using <CFCONTENT> to make the page output its contents as a .gif or .jpg instead of HTML. This would allow server-side processing to be used to choose an image without the user having to experience a page reload. While I haven't seen any practical uses for this yet, I'm hoping this suggestion will whet enough appetites that I might hear ideas from others.

Now all you have to do is pass on the variables that were passed to your error handler script to your ColdFusion page as follows:

errImage = new Image();
errImage.src = 'JSErrs.cfm?url=' + escape(errUrl)
+ '&msg=' + escape(msg) + '&line=' + line;

You can then use this ColdFusion page to send yourself an e-mail or log the error. I generally put a <cfmail> tag in mine and have the JavaScript errors e-mailed to myself. If you make sure that you call a ColdFusion page that's in the same application as the executing script (I always do), you can also send yourself CGI and session variables. Your URL variables will automatically be included in the URL passed to your ColdFusion page. Listing 3 shows a basic script that could be used as the ColdFusion page to handle the JavaScript error. If you aren't using CF5 or above, you can manually loop through the session and URL variables (as shown in Part 2 of Charlie's series [December 2000]) or you can use the CF_Dump custom tag that should still be available in the Macromedia Developer's Exchange.

All this means that you can be notified about a JavaScript error without the user's even knowing that the error occurred. This makes debugging easier for you and a better experience for your user. Remember to test any code that deals with user input carefully and to use try/catch blocks around it so you can tell your user about these errors. If they're unable to submit a form and don't know why, they'll find it very frustrating.

Summary
In this article I covered some ways to detect and prevent Java-Script errors. I also addressed handling errors in JavaScript. Finally, I covered how to send information about JavaScript errors on your page to ColdFusion and showed how to e-mail those errors to yourself. Hopefully this will help you deal with potential JavaScript errors in your current and future development. Good luck!

More Stories By Steve Bryant

Steve Bryant is the founder and CEO of Bryant Web Consulting LLC (www.bryantwebconsulting.com) and teaches ColdFusion at Breakaway Interactive (www.breakawayinteractive.com). He got his BA in philosophy at Oklahoma State University. Steve, one of the top ColdFusion developers in the country, still has no idea how that led to a career in Web development. Steve blogs at steve.coldfusionjournal.com as one of CFDJ's published bloggers.

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