- What is Express Middleware? 2:22
- Middleware Sequence and Routing 5:11
- Review: Middleware Sequence and Routing 5 questions
- The `next` function and a Closer Look at Middleware 4:45
- Handling Errors in an Express App 3:56
- Review: `next` and Handling Errors 5 questions
- Error Handling Middleware 5:20
- Handling 404 Errors 2:34
- Review: Error Handling and 404s 5 questions
Preview
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Handling Errors in an Express App
3:56Handling errors is an important (and easy to overlook) topic any program should address. Learn how to handle them in Express.
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.
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