javascript node.js next()

Question

This appears to be a variable naming convention in Node.js control-flow code, where a reference to the next function to execute is given to a callback for it to kick-off when it's done.

See, for example, the code samples here:

• http://blog.mixu.net/2011/02/02/essential-node-js-patterns-and-snippets/

Let's look at the example you posted:

function loadUser(req, res, next) {
  if (req.session.user_id) {
    User.findById(req.session.user_id, function(user) {
      if (user) {
        req.currentUser = user;
        return next();
      } else {
        res.redirect('/sessions/new');
      }
    });
  } else {
    res.redirect('/sessions/new');
  }
}

app.get('/documents.:format?', loadUser, function(req, res) {
  // ...
});

The loadUser function expects a function in its third argument, which is bound to the name next. This is a normal function parameter. It holds a reference to the next action to perform and is called once loadUser is done (unless a user could not be found).

There's nothing special about the name next in this example; we could have named it anything.

It is naming convention used when passing callbacks in situations that require serial execution of actions, e.g. scan directory -> read file data -> do something with data. This is in preference to deeply nesting the callbacks. The first three sections of the following article on Tim Caswell's HowToNode blog give a good overview of this:

http://howtonode.org/control-flow

Also see the Sequential Actions section of the second part of that posting:

http://howtonode.org/control-flow-part-ii

It's basically like a callback that express.js use after a certain part of the code is executed and done, you can use it to make sure that part of code is done and what you wanna do next thing, but always be mindful you only can do one res.send in your each REST block...

So you can do something like this as a simple next() example:

app.get("/", (req, res, next) => {
  console.log("req:", req, "res:", res);
  res.send(["data": "whatever"]);
  next();
},(req, res) =>
  console.log("it's all done!");
);

It's also very useful when you'd like to have a middleware in your app...

To load the middleware function, call app.use(), specifying the middleware function. For example, the following code loads the myLogger middleware function before the route to the root path (/).

var express = require('express');
var app = express();

var myLogger = function (req, res, next) {
  console.log('LOGGED');
  next();
}

app.use(myLogger);

app.get('/', function (req, res) {
  res.send('Hello World!');
})

app.listen(3000);