Using onError
Here are more constructs not available in PHP. Using either the
onError
event, or a combination of
the try
and catch
keywords, you can catch JavaScript
errors and deal with them yourself.
Events are actions that can be detected by JavaScript. Every
element on a web page has certain events that can trigger JavaScript
functions. For example, the onClick
event of a button element can be set to call a function and make it run
whenever a user clicks on the button.
Example 15-11
illustrates how to use the onError
event.
<script> onerror = errorHandler document.writ("Welcome to this website") // Deliberate error function errorHandler(message, url, line) { out = "Sorry, an error was encountered.\n\n"; out += "Error: " + message + "\n"; out += "URL: " + url + "\n"; out += "Line: " + line + "\n\n"; out += "Click OK to continue.\n\n"; alert(out); return true; } </script>
The first line of this script tells the error event to use the new
errorHandler
function from now on.
This function takes three parameters, a message
, a url
and a line
number, so it’s a simple matter to
display all these in an alert pop up.
Then, to test the new function, a syntax error is deliberately
placed in the code with a call to document.writ
instead of document.write
(the final e
is missing). Figure 15-1 shows the result
of running this script in a browser. Using onError
this way can also be quite useful
during the debugging process.
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