onError

Executes JavaScript code when an error event occurs; that is, when the loading of a document or image causes an error.

Event handler for
Image, Window

Implemented in
Navigator 3.0

Syntax

onError="handlerText"

Parameters

handlerText
JavaScript code or a call to a JavaScript function.

Description

An error event occurs only when a JavaScript syntax or runtime error occurs, not when a browser error occurs. For example, if you try set window.location.href='notThere.html' and notThere.html does not exist, the resulting error message is a browser error message; therefore, onError would not intercept that message. However, an error event is triggered by a bad URL within an IMG tag or by corrupted image data.

window.onerror applies only to errors that occur in the window containing window.onerror, not in other windows.

onError can be any of the following:

	null to suppress all JavaScript error dialogs. Setting `window.onerror` to null means your users won't see JavaScript errors caused by your own code.
	The name of a function that handles errors (arguments are message text, URL, and line number of the offending line). To suppress the standard JavaScript error dialog, the function must return true. See Example 3 below.
	A variable or property that contains null or a valid function reference.

If you write an error-handling function, you have three options for reporting errors:

	Trace errors but let the standard JavaScript dialog report them (use an error handling function that returns false or does not return a value)
	Report errors yourself and disable the standard error dialog (use an error handling function that returns true)
	Turn off all error reporting (set the onError event handler to null)

Event properties used

type
Indicates the type of event.

target
Indicates the object to which the event was originally sent.

Examples

Example 1: Null event handler. In the following IMG tag, the code onError="null" suppresses error messages if errors occur when the image loads.

<IMG NAME="imageBad1" SRC="corrupt.gif" ALIGN="left" BORDER="2"
   onError="null">

Example 2: Null event handler for a window. The onError event handler for windows cannot be expressed in HTML. Therefore, you must spell it all lowercase and set it in a SCRIPT tag. The following code assigns null to the onError handler for the entire window, not just the Image object. This suppresses all JavaScript error messages, including those for the Image object.

<SCRIPT>
window.onerror=null
</SCRIPT>
<IMG NAME="imageBad1" SRC="corrupt.gif" ALIGN="left" BORDER="2">

However, if the Image object has a custom onError event handler, the handler would execute if the image had an error. This is because window.onerror=null suppresses JavaScript error messages, not onError event handlers.

<SCRIPT>
window.onerror=null
function myErrorFunc() {
   alert("The image had a nasty error.")
}
</SCRIPT>
<IMG NAME="imageBad1" SRC="corrupt.gif" ALIGN="left" BORDER="2"
   onError="myErrorFunc()">

In the following example, window.onerror=null suppresses all error reporting. Without onerror=null, the code would cause a stack overflow error because of infinite recursion.

<SCRIPT>
window.onerror = null;
function testErrorFunction() {
   testErrorFunction();
}
</SCRIPT>
<BODY onload="testErrorFunction()">
test message
</BODY>

Example 3: Error handling function. The following example defines a function, myOnError, that intercepts JavaScript errors. The function uses three arrays to store the message, URL, and line number for each error. When the user clicks the Display Error Report button, the displayErrors function opens a window and creates an error report in that window. Note that the function returns true to suppress the standard JavaScript error dialog.

<SCRIPT>
window.onerror = myOnError

msgArray = new Array()
urlArray = new Array()
lnoArray = new Array()

function myOnError(msg, url, lno) {
   msgArray[msgArray.length] = msg
   urlArray[urlArray.length] = url
   lnoArray[lnoArray.length] = lno
   return true
}

function displayErrors() {
   win2=window.open('','window2','scrollbars=yes')
   win2.document.writeln('<B>Error Report</B><P>')

   for (var i=0; i < msgArray.length; i++) {
      win2.document.writeln('<B>Error in file:</B> ' + urlArray[i] + '<BR>')
      win2.document.writeln('<B>Line number:</B> ' + lnoArray[i] + '<BR>')
      win2.document.writeln('<B>Message:</B> ' + msgArray[i] + '<P>')
   }
   win2.document.close()
}
</SCRIPT>

<BODY onload="noSuchFunction()">
<FORM>
<BR><INPUT TYPE="button" VALUE="This button has a syntax error"
   onClick="alert('unterminated string)">

<P><INPUT TYPE="button" VALUE="Display Error Report"
   onClick="displayErrors()">
</FORM>

This example produces the following output:

Error Report

Error in file: file:///c%7C/temp/onerror.html
Line number: 34
Message: unterminated string literal

Error in file: file:///c%7C/temp/onerror.html
Line number: 34
Message: missing ) after argument list

Error in file: file:///c%7C/temp/onerror.html
Line number: 30
Message: noSuchFunction is not defined

Example 4: Event handler calls a function. In the following IMG tag, onError calls the function badImage if errors occur when the image loads.

<SCRIPT>
function badImage(theImage) {
   alert('Error: ' + theImage.name + ' did not load properly.')
}
</SCRIPT>
<FORM>
<IMG NAME="imageBad2" SRC="orca.gif" ALIGN="left" BORDER="2"
   onError="badImage(this)">
</FORM>

Event handler for	`Image`, `Window`
Implemented in	Navigator 3.0

type	Indicates the type of event.
target	Indicates the object to which the event was originally sent.

onError

Syntax

Parameters

Description

Event properties used

Examples

See also