node.js - What does double parentheses mean in a require

Question

I know what this require statement does.

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

But sometimes i have seen two parentheses after the require.

var routes = require('./routes')(app);

Q) What does this mean, and how does it work?

Answer 1

This is a pattern in which the module.exports of the module you are requiring is set to a function. Requiring that module returns a function, and the parentheses after the require evaluates the function with an argument.

In your example above, your ./routes/index.js file would look something like the following:

module.exports = function(app) {
  app.get('/', function(req, res) {

  });
  // ...
};

This pattern is often used to pass variables to modules, as can bee seen above with the app variable.