CHAPTER 11
Exceptions and Error Handling
As much as we would all like to live in an error-free world, we don’t have that luxury.
Even the most trivial applications are subject to errors arising from conditions you
didn’t anticipate. The first step to writing robust, high-quality software is acknowl‐
edging that it will have errors. The second step is anticipating those errors and han‐
dling them in a reasonable fashion.
Exception handling is a mechanism that came about to deal with errors in a controlled
fashion. It’s called exception handling (as opposed to error handling) because it’s
meant to deal with exceptional circumstances—that is, not the errors you anticipate,
but the ones you don’t.
The line between anticipated errors and unanticipated errors (exceptions) is a blurry
one that is very much situational. An application that is designed to be used by the
general, untrained public may anticipate a lot more unpredictable behavior than an
application designed to be used by trained users.
An example of an anticipated error is someone providing an invalid email address on
a form: people make typos all the time. An unanticipated error might be running out
of disk space, or a usually reliable service being unavailable.
The Error Object
JavaScript has a built-in
Error
object, which is convenient for any kind of error han‐
dling (exceptional or anticipated). When you create an instance of
Error
, you can
provide an error message:
const
err
=
new
Error
(
'invalid email'
);
Creating an
Error
instance doesn’t, by itself, do anything. What it does is give you
something that can be used to communicate errors. Imagine a function that validates
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