spacer
Yehuda Shiran December 10, 1999
Error Handling
Tips: December 1999

Yehuda Shiran, Ph.D.
Doc JavaScript

Developer News
OpenOffice 3.2 Lands Amid Critical Changes
Red Hat, IBM Firmly in KVM Virtualization Camp
Red Hat Talks Up Open Source Cloud Plans

 As explained in Exception Handling (December 9, 1999), you want to avoid system messages because they don't make much sense for the average user. An alternative to adding the try...catch blocks is to redefine the window's onerror property:

window.onerror = myError;

You then define the myError function as you wish. For example:

function myError(message, url, line){
  alert("Let me handle it all by myself");
  return true;
}

The error handling function has three parameters: message that describes the error, url that holds the URL of the file, and line which points to the culprit line in the file. Your handling of the error may or may not use these parameters. In the example above, we just print a message, avoiding these parameters altogether.

The tricky part is whether the system does or does not show its error in addition to your treatment. You can control it by returning either a true or a false value. When the error function returns a true value, the system will not display any alert boxes of its own. When the function returns a false value, the system will display its original error messages after it exits the error handling function.

The following script shows the false-returning version. Try it:

<HTML>
<HEAD>
<TITLE> onerror Example 2 </TITLE>
<SCRIPT LANGUAGE="JavaScript">
window.onerror = myError;
function myError(message, url, line){
  alert("Let me handle it first...");
  return false;
}
</SCRIPT>
</HEAD>
<BODY>
<IMG SRC="photo32.gif">
<SCRIPT LANGUAGE="JavaScript1.2">
<!--
  for (image=0; image < document.images.length + 2; image++) {
    alert("The width of image " + image + " is " + document.images[image].width);
  }
// -->
</SCRIPT>
</BODY>
</HTML>

Here is the true-returning version. Try it:

<HTML>
<HEAD>
<TITLE> onerror Example 1</TITLE>
<SCRIPT LANGUAGE="JavaScript">
window.onerror = myError;
function myError(message, url, line){
  alert("Let me handle it all by myself");
  return true;
}
</SCRIPT>
</HEAD>
<BODY>
<IMG SRC="photo32.gif">
<SCRIPT LANGUAGE="JavaScript1.2">
<!--
  for (image=0; image < document.images.length + 2; image++) {
    alert("The width of image " + image + " is " + document.images[image].width);
  }
// -->
</SCRIPT>
</BODY>
</HTML>

The exception shown above is triggered by accessing a non-existent image. You can learn more about exception handling in Exception Handling (December 9, 1999) and in Column 38, Exception Handling.

People who read this tip also read these tips:

Look for similar tips by subject:


The Network for Technology Professionals

Search:

About Internet.com

Legal Notices, Licensing, Permissions, Privacy Policy.
Advertise | Newsletters | E-mail Offers

Solutions

Whitepapers and eBooks

Article: Adobe Helps PHP Developers Create Rich Internet Applications
Case Study: Microsoft IT Strengthens Security with Data Loss Prevention Solution
Article: Reduce Your Infrastructure Costs with Microsoft System Center
Intel Brief: Five-Star IT Support--Intel Core 2 processor with vPro Delivers ROI of 524 Percent
Article: Security Enhancements Abound in Windows Server 2008
Article: Enterprise Social Computing with Microsoft SharePoint 2010
Intel Whitepaper: Implementing Intel vPro Technology to Drive Down Client Management Costs
 Microsoft Whitepaper: Improve Communications and Collaboration
Intel Article: Intel Core i5 vPro Brings Intelligence to the Processor
Microsoft Personalized Whitepapers and Resources for You
Microsoft Articles: Visual Studio 2010
Adobe Article: Java Developers Finding a Home at Adobe Flex
Microsoft Whitepaper: Make Better Decisions - SQL Server 2008 Business Intelligence.
MORE WHITEPAPERS, EBOOKS, AND ARTICLES

Webcasts

Video: How System Center Helps Reduce IT Costs
IBM Cloud Computing for Developers Virtual Event, April 6-8
Microsoft Video: OCS and Exchange: Platform Futures
MORE WEBCASTS, PODCASTS, AND VIDEOS

Downloads and eKits

HP PartnerONE | SolutionsINFINITE Visit us at hp.com/partners/us
Iron Speed Designer Application Generator
 MORE DOWNLOADS, EKITS, AND FREE TRIALS

Tutorials and Demos

Learn Unified Communications
Learn SQL Server 2008
Learn Windows Server 2008 R2
Internet.com Hot List: Get the Inside Scoop on IT and Developer Products
Learn Forefront
 Learn System Center
All About Botnets
Learn SharePoint
MORE TUTORIALS, DEMOS AND STEP-BY-STEP GUIDES
webref The latest from WebReference.com Browse >
Search Engine Optimization: Selecting and Embedding Keywords · Are Google's Language Translation Web Services Ready for Prime Time? · Installing and Using Meeplace, the Business Review CMS
Sitemap · Experts · Tools · Services · Email a Colleague · Contact FREE Newsletters > 
 The latest from internet.com
IBM DB2 10 for z/OS: Justifying the Upgrade · Living La Vida Colo: Choosing the Right Colocation Facility · FTC Concerns over Social Media Privacy Linger