What does double parentheses mean in a require

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?

That mean that behind that, there is a function that is exported using `module.exports`:

module.exports = function(app) {
app.get("/", function(req, res){});
}

See also

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Sidenote:

You could create function on the fly:

A.js

module.exports = function(data) {
return function(req, res, next) {
// Do something with data
next();

}

main.js

...
app.use(require("A")(data));
...

Well, [require](

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) 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);

This is a pattern in which the [`module.exports`][1] of the module you are requiring is set to a function. [Requiring][2] 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.


[1]:

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[2]:

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