home / experts / javascript / column38 |
   |
|
|
|
Let's use now the exception handling functions from the previous page to handle a more realistic example. Suppose you have a page with one <IMG> tag. The program accepts a few attribute names and extracts the value of the corresponding image object properties. The possible exceptions caused by accessing incorrect attribute names and out-of-bound array elements. Here is the complete code:
<HTML>
<HEAD>
<TITLE> example 1 </TITLE>
</HEAD>
<BODY>
<IMG SRC="doc.gif">
<SCRIPT LANGUAGE="JavaScript1.2">
<!--
function createException(msgNum, msgText) {
this.messageNumber = msgNum;
this.messageText = msgText;
}
function triggerException(messageNum, messageTxt) {
exceptionObj = new createException(messageNum, messageTxt);
throw exceptionObj;
}
function raiseException(num, message) {
try {
triggerException(num, message);
}
catch (exceptionObj) {
if (exceptionObj instanceof createException) {
alert("Call the programmer ASAP")
}
else {
throw exceptionObj;
}
}
}
fieldNames = new Array("border", "complete", "height", "hspace",
"lowsrc", "name", "src", "vspace", "width");
// (The above two lines should be joined as one line.
// They have been split for formatting purposes.)
try {
for (image=0; image < document.images.length + 1; image++) {
for (i=0; i < fieldNames.length; i++) {
if (document.images[image] == null) triggerException( 98,
"illegal access to images array");
// (The above two lines should be joined as one line.
// They have been split for formatting purposes.)
if (eval("document.images[image]." + fieldNames[i]) != null)
alert(fieldNames[i] + " is " + eval("document.images[image]."
+ fieldNames[i]))
// (The above three lines should be joined as one line.
// They have been split for formatting purposes.)
else triggerException(i, "field name " + fieldNames[i] + "
is not legal");
// (The above two lines should be joined as one line.
// They have been split for formatting purposes.)
}
}
}
catch (exceptionObj) {
if (exceptionObj instanceof createException) {
if (exceptionObj.messageNumber > 0 && exceptionObj.messageNumber
< 9) alert("Problem in property: " + exceptionObj.messageText)
// (The above two lines should be joined as one line.
// They have been split for formatting purposes.)
else if (exceptionObj.messageNumber < = 98 &&
exceptionObj.messageNumber >= 97) alert("Problem in images array: "
+ exceptionObj.messageText)
// (The above three lines should be joined as one line.
// They have been split for formatting purposes.)
else raiseException(99, "Catch All");;
}
}
//raiseException(1, "Doc JavaScript Column 36");
// -->
</SCRIPT>
</BODY>
</HTML>We first define the fieldNames array. The try block consists of two nested loops. The outer one iterates through the page's images:
for (image=0; image < document.images.length + 1; image++) {The inner loop iterates through the fieldNames elements:
for (i=0; i < fieldNames.length; i++) {Inside the loop, we check for two exceptions. The first check verifies that there is no attempt to access an out-of-bound array element. We trigger an exception when there is an invalid attempt:
if (document.images[image] == null) triggerException( 98, "illegal access to images array");The second check prints the value of the relevant object's property or triggers an exception when the field name does not match any of the object's:
if (eval("document.images[image]." + fieldNames[i]) != null) alert(fieldNames[i] +
" is " + eval("document.images[image]." + fieldNames[i]))
// (The above two lines should be joined as one line.
// They have been split for formatting purposes.)
else triggerException(i, "field name " + fieldNames[i] + " is not legal");The catch block decipher the exception objects. First, we check if the object is of the right type:
if (exceptionObj instanceof createException) {
As explained earlier, this check is of utmost importance for distinguishing between JavaScript errors and user errors. Once we know it is a user-thrown exception, we check if the error number is between 1 and 9, and print an appropriate message.
if (exceptionObj.messageNumber > 0 && exceptionObj.messageNumber < 9) alert("Problem
in property: " + exceptionObj.messageText)
// (The above two lines should be joined as one line.
// They have been split for formatting purposes.)
We check if the error number is 98 or 97 and then print a message related to the exception:
else if (exceptionObj.messageNumber < = 98 && exceptionObj.messageNumber >= 97)
alert("Problem in images array: " + exceptionObj.messageText)
// (The above two lines should be joined as one line.
// They have been split for formatting purposes.)
If the exception object does not match any of the error types, we raise a general exception to be handled by the operating system:
else raiseException(99, "Catch All")Always remember that if you don't handle the exception, someone else will. The operation system handles all unattended exceptions.
Produced by Yehuda Shiran and Tomer Shiran
Legal Notices, Licensing, Reprints, Permissions, Privacy Policy.
Advertise | Newsletters | Shopping | E-mail Offers | Freelance Jobs 
Created: April 26, 1999
Revised: April 26, 1999
URL: http://www.webreference.com/js/column38/realexample.html
