Wanted to clear up something in this course that I did not fully understand.

Question

One thing I did not fully understand was the error handling structure that is used through out this course.
javascript
var err = new Error('You must be logged in to view this page.');
    err.status = 401;
    return next(err);

Create a new error object, give it a status then pass it along as an argument to the next method of the middleware. Ok I get that, but now there are only 2 things that I was confused about in that process.
For what purpose did you pass the error object into the next method?
How is the error being pushed to the front end of error.pug?
```javascript
extends layout
block content
  .main.container
    .row.text-xs-center
      .col-md-6.col-md-offset-3.p-t-2
        i.icn-person.material-icons error
        p.lead.m-t-2= message 
```
I see the message variable there but I am not sure how that ties in with there errors we create. Also I know that variables being passed into pug have the hashtag and 2 parentheses like so #{name} which the message variable does not. Other then that those 2 things are the only part of this course I did not completely understand, everything else was a breeze thanks @Dave McFarland (https://teamtreehouse.com/davemcfarland) !

Joel Kraft · Accepted Answer

Hi Lucas,
In Expressjs middleware, calling the next() function passes control to the next middleware. However, if next() is called with an object parameter (i.e. next(err)), control skips all remaining middleware, and  will trigger the first error handler to run.
How does Express know the difference between middleware and error handlers? Middleware has 3 arguments in the function signature

function(req, res, next) { /* ... */ }

and error handlers have 4

function(err, req, res, next) { /* ... */ }

So in app.js, the error handler is down toward the bottom, and it looks like this:

// error handler
// define as the last app.use callback
app.use(function(err, req, res, next) {
  res.status(err.status || 500);
  res.render('error', {
    message: err.message,
    error: {}
  });
});

This is where the error lands, and you can see the handler is rendering 'error', which is error.pug.
Here is a video that covers this topic (https://teamtreehouse.com/library/error-handling-in-express), albeit with JSON instead of renderable templates like error.pug.
Also, here is the documentation (https://expressjs.com/en/guide/error-handling.html) if you'd like to read more about it.

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Lucas Santos

Lucas Santos

Wanted to clear up something in this course that I did not fully understand.

1 Answer

Joel Kraft

Joel Kraft

Lucas Santos

Lucas Santos