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.

Answer 2

Well, require is a function provided by Node.js that basically loads a module for you and it returns whatever you expose in the module that you loaded.

If what you expose (through the use of module.exports) in a given module is a function, then that is what requires returns. For instance.

//moduleX.js module.exports = function(){   return "Helo World"; } 

Then if you require it, you get a function back

var f = require('./moduleX'); console.log(f()); //hello world 

Of course, you could invoke the function directly once you require it.

var greet = require('./moduleX')(); console.log(greet);