Welcome!

You will be redirected in 30 seconds or close now.

ColdFusion Authors: Yakov Fain, Jeremy Geelan, Maureen O'Gara, Nancy Y. Nee, Tad Anderson

Related Topics: ColdFusion

ColdFusion: Article

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.

Comments (1) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
10/17/02 03:31:00 PM EDT
@ThingsExpo Stories
To get the most out of their data, successful companies are not focusing on queries and data lakes, they are actively integrating analytics into their operations with a data-first application development approach. Real-time adjustments to improve revenues, reduce costs, or mitigate risk rely on applications that minimize latency on a variety of data sources. Jack Norris reviews best practices to show how companies develop, deploy, and dynamically update these applications and how this data-first...
Intelligent Automation is now one of the key business imperatives for CIOs and CISOs impacting all areas of business today. In his session at 21st Cloud Expo, Brian Boeggeman, VP Alliances & Partnerships at Ayehu, will talk about how business value is created and delivered through intelligent automation to today’s enterprises. The open ecosystem platform approach toward Intelligent Automation that Ayehu delivers to the market is core to enabling the creation of the self-driving enterprise.
Internet-of-Things discussions can end up either going down the consumer gadget rabbit hole or focused on the sort of data logging that industrial manufacturers have been doing forever. However, in fact, companies today are already using IoT data both to optimize their operational technology and to improve the experience of customer interactions in novel ways. In his session at @ThingsExpo, Gordon Haff, Red Hat Technology Evangelist, shared examples from a wide range of industries – including en...
Consumers increasingly expect their electronic "things" to be connected to smart phones, tablets and the Internet. When that thing happens to be a medical device, the risks and benefits of connectivity must be carefully weighed. Once the decision is made that connecting the device is beneficial, medical device manufacturers must design their products to maintain patient safety and prevent compromised personal health information in the face of cybersecurity threats. In his session at @ThingsExpo...
"We're a cybersecurity firm that specializes in engineering security solutions both at the software and hardware level. Security cannot be an after-the-fact afterthought, which is what it's become," stated Richard Blech, Chief Executive Officer at Secure Channels, in this SYS-CON.tv interview at @ThingsExpo, held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that Massive Networks will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Massive Networks mission is simple. To help your business operate seamlessly with fast, reliable, and secure internet and network solutions. Improve your customer's experience with outstanding connections to your cloud.
SYS-CON Events announced today that Grape Up will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct. 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Grape Up is a software company specializing in cloud native application development and professional services related to Cloud Foundry PaaS. With five expert teams that operate in various sectors of the market across the U.S. and Europe, Grape Up works with a variety of customers from emergi...
Detecting internal user threats in the Big Data eco-system is challenging and cumbersome. Many organizations monitor internal usage of the Big Data eco-system using a set of alerts. This is not a scalable process given the increase in the number of alerts with the accelerating growth in data volume and user base. Organizations are increasingly leveraging machine learning to monitor only those data elements that are sensitive and critical, autonomously establish monitoring policies, and to detect...
Everything run by electricity will eventually be connected to the Internet. Get ahead of the Internet of Things revolution and join Akvelon expert and IoT industry leader, Sergey Grebnov, in his session at @ThingsExpo, for an educational dive into the world of managing your home, workplace and all the devices they contain with the power of machine-based AI and intelligent Bot services for a completely streamlined experience.
Because IoT devices are deployed in mission-critical environments more than ever before, it’s increasingly imperative they be truly smart. IoT sensors simply stockpiling data isn’t useful. IoT must be artificially and naturally intelligent in order to provide more value In his session at @ThingsExpo, John Crupi, Vice President and Engineering System Architect at Greenwave Systems, will discuss how IoT artificial intelligence (AI) can be carried out via edge analytics and machine learning techn...
When shopping for a new data processing platform for IoT solutions, many development teams want to be able to test-drive options before making a choice. Yet when evaluating an IoT solution, it’s simply not feasible to do so at scale with physical devices. Building a sensor simulator is the next best choice; however, generating a realistic simulation at very high TPS with ease of configurability is a formidable challenge. When dealing with multiple application or transport protocols, you would be...
With tough new regulations coming to Europe on data privacy in May 2018, Calligo will explain why in reality the effect is global and transforms how you consider critical data. EU GDPR fundamentally rewrites the rules for cloud, Big Data and IoT. In his session at 21st Cloud Expo, Adam Ryan, Vice President and General Manager EMEA at Calligo, will examine the regulations and provide insight on how it affects technology, challenges the established rules and will usher in new levels of diligence a...
An increasing number of companies are creating products that combine data with analytical capabilities. Running interactive queries on Big Data requires complex architectures to store and query data effectively, typically involving data streams, an choosing efficient file format/database and multiple independent systems that are tied together through custom-engineered pipelines. In his session at @BigDataExpo at @ThingsExpo, Tomer Levi, a senior software engineer at Intel’s Advanced Analytics ...
In the enterprise today, connected IoT devices are everywhere – both inside and outside corporate environments. The need to identify, manage, control and secure a quickly growing web of connections and outside devices is making the already challenging task of security even more important, and onerous. In his session at @ThingsExpo, Rich Boyer, CISO and Chief Architect for Security at NTT i3, discussed new ways of thinking and the approaches needed to address the emerging challenges of security i...
SYS-CON Events announced today that Dasher Technologies will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Dasher Technologies, Inc. ® is a premier IT solution provider that delivers expert technical resources along with trusted account executives to architect and deliver complete IT solutions and services to help our clients execute their goals, plans and objectives. Since 1999, we'v...
There is only one world-class Cloud event on earth, and that is Cloud Expo – which returns to Silicon Valley for the 21st Cloud Expo at the Santa Clara Convention Center, October 31 - November 2, 2017. Every Global 2000 enterprise in the world is now integrating cloud computing in some form into its IT development and operations. Midsize and small businesses are also migrating to the cloud in increasing numbers. Companies are each developing their unique mix of cloud technologies and service...
SYS-CON Events announced today that IBM has been named “Diamond Sponsor” of SYS-CON's 21st Cloud Expo, which will take place on October 31 through November 2nd 2017 at the Santa Clara Convention Center in Santa Clara, California.
SYS-CON Events announced today that Datera, that offers a radically new data management architecture, has been named "Exhibitor" of SYS-CON's 21st International Cloud Expo ®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Datera is transforming the traditional datacenter model through modern cloud simplicity. The technology industry is at another major inflection point. The rise of mobile, the Internet of Things, data storage and Big...
SYS-CON Events announced today that Akvelon will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Akvelon is a business and technology consulting firm that specializes in applying cutting-edge technology to problems in fields as diverse as mobile technology, sports technology, finance, and healthcare.
WebRTC is great technology to build your own communication tools. It will be even more exciting experience it with advanced devices, such as a 360 Camera, 360 microphone, and a depth sensor camera. In his session at @ThingsExpo, Masashi Ganeko, a manager at INFOCOM Corporation, will introduce two experimental projects from his team and what they learned from them. "Shotoku Tamago" uses the robot audition software HARK to track speakers in 360 video of a remote party. "Virtual Teleport" uses a...