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Handling syntax errors

The

onerror

event handler isn’t just good for dealing with exceptions; it is also the only way to deal

with syntax errors.

To do so, the event handler must be the first code that appears in the page. Why the first? If a syntax

error occurs before the event handler has been set up, that event handler will never be set up.

Remember, a syntax error completely stops code execution. Consider the following example:

<html>

<head>

<title>OnError Example</title>

alert(“Syntax error. “;

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

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

“\nLine Number: “ + sLine);

return true;

}

</script>

</head>

</body>

</html>

Because the highlighted line (which is a syntax error) occurs before the

onerror

event handler is

assigned, the browser reports the error directly. The code immediately following the error is not inter-

preted (because the thread is exited) so when the

load

event fires and the

nonExistentFunction()

called, the browser reports that error as well. If this page is rewritten to place the

onerror

event-handler

assignment before the syntax error, two alerts are displayed: one showing the syntax error and one

showing the exception.

<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;

}

alert(“Syntax error. “;

</script>

</head>

</body>

</html>

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