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Extracting error information

In this example, an exception occurs when an attempt is made to call

nonExistentFunction()

which doesn’t exist. When this happens, the alert is displayed containing the message,

“An error

occurred.”

Unfortunately, the browser ’s error message is also displayed. To hide the error from the

browser (and thus prevent it from reporting the error), the

onerror

event handler must return a value

true

<html>

<head>

<title>OnError Example</title>

window.onerror = function () {

alert(“An error occurred. “);

return true;

}

</script>

</head>

</body>

</html>

Extracting error information

Simply knowing that an error occurred is of little use to a programmer without the error details.

Fortunately, the

onerror

event handler provides three pieces of information to identify the exact

nature of the error:

Error message — The same message that the browser would display for the given error

URL — The file in which the error occurred

Line number — The line number in the given URL that caused the error

This information is passed as three parameters into the

onerror

event handler and can be accessed

like this:

<html>

<head>

<title>OnError Example</title>

window.onerror = function (sMessage, sUrl, sLine) {

alert(“An error occurred:\n” + sMessage + “\nURL: “ + sUrl +

“\nLine Number: “ + sLine);

return true;

}

</script>

</head>

</body>

</html>

Using this code, it is possible to create custom JavaScript error dialogs that mimic the functionality of

browser-error dialogs.

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