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Remember back at
the beginning of the course,

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when we made our first request of the app.

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The client received a 404
error back from our route.

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This meant, Express didn't have
the page that we requested.

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More specifically,
it meant that there was no get route for

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our app matching the URL we requested.

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Applications always need a way
of handling errors like this.

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Other errors might occur when
the app can't reach a database, or

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when a user mistype their password.

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Errors are an important tool for

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your user as they learn how
to interact with your app.

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They offer information about
the limits of how your app can be used.

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Errors also guide developers in
fixing bugs in an application.

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In Express, you can use the next
function to signal an error in your app.

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By passing an object as
a parameter to next,

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Express knows there is an error to handle.

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Express will then immediately
terminate and handle the error.

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Let's create our own error.

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We'll use the new error constructor,
And pass in a string.

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The string will serve as an error message.

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Normally, no, wouldn't be a good
error string in a real app,

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because it doesn't provide any
clues about what went wrong.

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But as you'll see soon there is a lot
of text when an error is thrown, and

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hopefully this text will be easy to
pick out when we view it on the browser.

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This is JavaScript's
native error constructor.

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If you haven't seen this syntax before or

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haven't seen it too often,
don't worry about it for now.

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It's enough to know that we're creating a
custom error object and storing it in ERR.

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Look in the teacher's notes if you'd
like to read more about JavaScript's

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error constructor.

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Let's pass in the error object as
an argument to the next function call.

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If I save it and refresh the browser,
there is a lot of text here.

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The error is thrown and at the top, you
can see the error message was outputted.

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The next line tells us that
the error was thrown from line 14.

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The second number is the column, or

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how many spaces from the first
position on line 14.

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Pinpointing where an error is in an app
can really speed up the process of

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fixing bugs.

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All of the lines after this indicate
deeper layers of the Express framework,

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and you probably won't
need to go into this.

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Sometimes though, if you really stumped,

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pasting this into a Google search
can get you closer to your answer.

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Let's check the console.

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And you can see Hello is logged out
just above where the error was thrown.

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World never gets logged, because
the program was interrupted by the error.

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Let's switch back to the browser.

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This output is coming from
Express's built in error handler.

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It has some good information in here,
but the formatting has some room for

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improvement.

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More readable error messages can
make development an easier process.

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Express offers a way to write
our own custom error handlers,

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where we can format the errors
to be much more readable.

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I'll show yo how to do that next.