When to use () [i.e. function call] inside app.use?

Hi aviralkulshreshtha,

jsonParser is 3rd-party middleware from the body-parser library. Very often, 3rd-party middleware will take the form of a function that returns a middleware function. That's what's happening here, something along the lines of the following:

function thirdPartyMiddleware () {
  // set up middleware function
  // ...
  return function( req, res, next ) {
    // process req and/or res
    next();
  }
}

Then, when this function is called, the inner function is passed to app.use:

app.use( thirdPartyMiddleware() );

A function that returns another function is sometimes referred to as a closure. We have a closure workshop that looks at this a little more closely, if you're interested:

• Understanding Closures in JavaScript

The final video of the workshop, we actually peek under the hood a 3rd-party Express middleware library!

Hope this helps!