Using try...catch
The try
and catch
keywords are more standard and more
flexible than the onError
technique
shown in the previous section. These keywords let you trap errors for a
selected section of code, rather than all scripts in a document.
However, they do not catch syntax errors, for which you need onError
.
The try...catch
construct is
supported by all major browsers and is handy when you want to catch a
certain condition that you are aware could occur in a specific part of
your code.
For example, in Chapter 18 we’ll be exploring
Ajax techniques that make use of the XMLHttpRequest
object. Unfortunately, this
isn’t available in the Internet Explorer browser (although it is in all
other major browsers). Therefore, we can use try
and catch
to trap this case and do something else
if the function is not available. Example 15-12 shows how.
<script> try { request = new XMLHTTPRequest() } catch(err) { // Use a different method to create an XML HTTP Request object } </script>
I won’t go into how we implement the missing object in Internet
Explorer here, but you can see how the system works. There’s also
another keyword associated with try
and catch
called finally
that is always executed, regardless of
whether an error occurs in the try
clause. To use it, just add something like the following statements
after a catch
statement:
finally { alert("The 'try' clause was encountered") }
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