Not able to receive error in express.js middleware function

Question

I'm trying to write an express middleware that handles errors. However, I'm not able to define a function with err as an argument. In the express documentation I read the following:

Define error-handling middleware functions in the same way as other middleware functions, except error-handling functions have four arguments instead of three: (err, req, res, next)

But when I define the following function, it seems like it's not invoked:

myFunction = (config) => {

  return myFunction = (err, req, res, next) {

    console.log(err)

  }

}

module.exports = myFunction

The function above isn't invoked when I specify app.use(myFunction(config)) from my express server, but if I remove the err argument it is. Does anyone know why that is the case?

Answer 1

You can get it to work. What you need to do is to specify 

app.use(myFunction(config)) 

below your routes in my server.