var express = require('express'); var app = express(), What is express()?? is it a method or a constructor? Where does it come from

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

This is how we create an express application. But what is this 'express()'? Is it a method or a constructor? Where does it come from??

Is it a method or a constructor?

Neither; it's a function, although if you said "method" I don't think anyone would give you a hard time.

A method is a function attached to an object. In JavaScript, methods are just mostly functions that you reference via object properties. (Update: As of ES2015, if you use method syntax to create them, they're slightly more than that because they have access to super.)

A constructor, in JavaScript, is a function you call via the new operator. Even though other functions may create things, we don't typically call them "constructors" to avoid confusion. Sometimes they may be "creator" or "builder" functions.

Where does it come from?

ExpressJS is a NodeJS module; express is the name of the module, and also the name we typically give to the variable we use to refer to its main function in code such as what you quoted. NodeJS provides the require function, whose job is to load modules and give you access to their exports. (You don't have to call the variable express, you can do var foo = require('express'); and use foo instead, but convention is that you'd use the module's name, or if only using one part of a module, to use the name of that part as defined by the module's documentation.)

The default export of express is a bit unusual in that it's a function that also has properties on it that are also functions (methods). That's perfectly valid in JavaScript,¹ but fairly unusual in some other languages. That's why you can create an Application object via express(), but also use express.static(/*...*/) to set up serving static files.

¹ In fact, it's completely normal. Functions have a couple of standard methods by default: call, apply, and toString for instance.

You’ll use Node’s require function to use the express module. require is similar to keywords like import or include in other languages. require takes the name of a package as a string argument and returns a package. There’s nothing special about the object that’s returned—it’s often an object, but it could be a function or a string or a number.

var express = require('express'); 

=> Requires the Express module just as you require other modules and and puts it in a variable.

var app = express(); 

=> Calls the express function "express()" and puts new Express application inside the app variable (to start a new Express application). It's something like you are creating an object of a class. Where "express()" is just like class and app is it's newly created object.

By looking the code of express below you are good to go what is really happening inside.

File 1: index.js

'use strict';

module.exports = require('./lib/express');

File 2 : lib/express.js

'use strict';

var EventEmitter = require('events').EventEmitter;
var mixin = require('merge-descriptors');
var proto = require('./application');
var Route = require('./router/route');
var Router = require('./router');
var req = require('./request');
var res = require('./response');

/**
 * Expose `createApplication()`.
 */

exports = module.exports = createApplication;

function createApplication() {
  var app = function(req, res, next) {
    app.handle(req, res, next);
  };

  mixin(app, EventEmitter.prototype, false);
  mixin(app, proto, false);

  app.request = { __proto__: req, app: app };
  app.response = { __proto__: res, app: app };
  app.init();
  return app;
}
exports.application = proto;
exports.request = req;
exports.response = res;    
exports.Route = Route;
exports.Router = Router;
});

How require works

When you call require('some_module') in node here is what happens:

1. if a file called some_module.js exists in the current folder node will load that, otherwise:

2. Node looks in the current folder for a node_modules folder with a some_module folder in it.

3. If it doesn't find it, it will go up one folder and repeat step 2

This cycle repeats until node reaches the root folder of the filesystem, at which point it will then check any global module folders (e.g. /usr/local/node_modules on Mac OS) and if it still doesn't find some_module it will throw an exception.

Ancient post. I think the original poster was confused about why the syntax to call the function exported by module express is

var app = express() 

instead of

var app = express.express()

To clarify: require() function does not create a reference to that 'module'. There's no such thing as reference to a module. There's only reference to thing(s) exported by a module.

require('xxx.js'), where the .js extension can be omitted, returns whatever is exported by that xxx.js file. If that xxx.js file exports an object, require('xxx.js') returns an object; if a function is exported, require('xxx.js') returns a function; if a single string is exported, require('xxx.js') returns a string...

If you check source code of file express.js, you will see that it exports a single function. So in

var express = require('express')

The first express is assigned whatever is exported by module express, which in this case happens to be a single function. express is a function, not a reference to a module. Hence on second row you just invoke that function:

var app = express()

Hope this helps!