javascript node.js next()


I see a lot of use next in node.js.

What is it, where does it come from? What does it do? Can I use it client side?

Sorry it's used for example here:

look for the loadUser function.

3/21/2011 10:44:16 PM

Accepted Answer

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:

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 {
  } else {

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.

11/17/2016 6:43:34 PM

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:

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

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