What is the function signature of express.js middlewere?

[jcubic]

On this page Using middleware the code look like this:

app.use((req, res, next) => {
  console.log('Time:', Date.now())
  next()
})

but on error handling page it's:

app.use((err, req, res, next) => {
  // logic
})

which one is correct? Why there are two signatures? Does express.js check the number of arguments? What if I want an error handler and don't use the last parameter because I don't use it?

[krzysdz]

Does express.js check the number of arguments?

Yes, express checks the number of arguments. 3 arguments - normal middleware, 4 - error handling middleware.

What if I want an error handler and don't use the last parameter because I don't use it?

Declare a function with 4 parameters and don't use the last one. If your linter/typechecker is complaining about it, prefixing it with _ may help.

app.use((err, req, res, _next) => {
   // handle the error without using next
}

[markcellus]

Yeah I've gotten bit by this issue when removing unused params. And it's not pretty because it's a very difficult bug to find. Express team should rethink this if possible.

[jcubic]

@markcellus Looking at the minimalistic API like express I don't think it would ever be possible, since it would be a huge breaking change. But maybe, who knows? Especially me since I'm not part of the dev team.