How to throw a 404 error in express.js?

Question

In app.js, I have

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('Not Found');
  err.status = 404;
  next(err);
});

so if I request some not exist url like http://localhost/notfound, above code will execute.

In exist url like http://localhost/posts/:postId, I would like to throw 404 error when access some not exist postId or deleted postId.

Posts.findOne({_id: req.params.id, deleted: false}).exec()
  .then(function(post) {
    if(!post) {
      // How to throw a 404 error, so code can jump to above 404 catch?
    }

Answer 1

In Express, a 404 isn't classed as an 'error', so to speak - the reasoning behind this is that a 404 isn't usually a sign that's something's gone wrong, it's just that the server couldn't find anything. Your best bet is to explicitly send a 404 in your route handler:

Posts.findOne({_id: req.params.id, deleted: false}).exec()
  .then(function(post) {
    if(!post) {
      res.status(404).send("Not found.");
    }

Or alternatively, if this feels like too much repeated code, you could always pull that code out into a function:

function notFound(res) {
    res.status(404).send("Not found.");
}

Posts.findOne({_id: req.params.id, deleted: false}).exec()
      .then(function(post) {
        if(!post) {
          notFound(res);
        }

I wouldn't recommend using a middleware in this situation solely because I feel like it makes the code less clear - the 404 is the direct result of the database code not finding anything, so it makes sense to have the response in the route handler.

Answer 2

I have the same app.js structure, and I solved this problem in this way in the route handler:

router.get('/something/:postId', function(req, res, next){
    // ...
    if (!post){
        next();
        return;
    }
    res.send('Post exists!');  // display post somehow
});

The next() function will call the next middleware which is the error404 handler if it is right after your routes in the app.js.

Answer 3

You can use this and the end of your routers.

app.use('/', my_router);
....
app.use('/', my_router);

app.use(function(req, res, next) {
        res.status(404).render('error/404.html');
    });