Understanding express() in Express.js: Is It a Method or Constructor? Where Does It Come From?

Table of Contents#

1. Where Does express() Come From?
   • 1.1 The Express.js Module
   • 1.2 The express Export: A Factory Function
2. Is express() a Method or Constructor?
   • 2.1 Defining Methods vs. Constructors
   • 2.2 Why express() Is Not a Method
   • 2.3 Why express() Is Not a Constructor (in the Traditional Sense)
   • 2.4 The Truth: express() as a Factory Function
3. What Does express() Return? The Express Application Object
   • 3.1 The Role of the app Object
   • 3.2 Key Methods of the app Object
4. Common Misconceptions
5. Conclusion
6. References

Where Does express() Come From?#

To understand express(), we first need to trace its origins. Let’s start with how you typically use Express in a project.

1.1 The Express.js Module#

Express.js is a Node.js module, which means it’s a reusable piece of code published on npm. To use it, you first install it via npm:

npm install express

Once installed, you “require” it in your code to access its functionality:

const express = require('express');

At this point, express is not a class, object, or method—it’s a function exported by the Express module. This function is the express() we’re trying to understand.

1.2 The express Export: A Factory Function#

The Express module’s main job is to export a function (let’s call it createApplication) that, when called, generates an Express application instance. This createApplication function is what you get when you require('express')—it’s the express() function in your code.

If you peek under the hood of the Express source code (available on GitHub), you’ll find that the main entry point (lib/express.js) exports this function directly:

// From Express's lib/express.js
var createApplication = require('./lib/application');
module.exports = createApplication;

Here, createApplication is the function responsible for creating new Express applications. When you write const express = require('express'), express becomes a reference to createApplication. Thus, express() is simply a call to this exported factory function.

Is express() a Method or Constructor?#

Now, let’s tackle the core question: What is express()? To answer this, we need to clarify the definitions of “method” and “constructor” in JavaScript.

2.1 Defining Methods vs. Constructors#

• Method: A function that is a property of an object. For example, array.push() is a method because push is a function attached to the Array prototype object.
• Constructor: A function designed to create and initialize objects. Constructors are typically called with the new keyword (e.g., new Date() or new Array()), and they return an instance of a class or object type.

2.2 Why express() Is Not a Method#

express() is not a method because it is not attached to an object. When you require('express'), you get a standalone function, not an object with properties. For example:

const express = require('express');
console.log(typeof express); // "function" (not an object)

Since express is a top-level function (not a property of another object), calling express() is not invoking a method—it’s calling a standalone function.

2.3 Why express() Is Not a Constructor (in the Traditional Sense)#

A traditional constructor is called with new to create an instance. For example:

class MyClass { /* ... */ }
const instance = new MyClass(); // Using `new` with a constructor

Express explicitly does not require new when calling express(). In fact, the official documentation and examples never use new express():

// Correct: No `new` keyword
const app = express(); 
 
// Incorrect (and unnecessary): `new` is not required
const app = new express(); // Works but is redundant

If express() were a constructor, the new keyword would be mandatory. Since it’s optional (and discouraged), express() is not a traditional constructor.

2.4 The Truth: express() as a Factory Function#

express() is best described as a factory function. A factory function is a function that returns a new object (or instance) without requiring the new keyword.

Internally, Express uses a constructor to create application instances, but this is hidden from the public API. Let’s see how:

The createApplication function (our express() function) delegates to an internal Application constructor. Here’s a simplified version of how it works:

// From Express's lib/application.js (simplified)
function Application() {
  // Initialize application settings, middleware, etc.
  this.settings = {};
  this.middleware = [];
  // ... more setup ...
}
 
// Add methods like get(), post(), listen() to the Application prototype
Application.prototype.get = function(path, handler) { /* ... */ };
Application.prototype.listen = function(port) { /* ... */ };
 
// From Express's lib/express.js (simplified)
function createApplication() {
  const app = new Application(); // Internal use of constructor
  return app; // Return the Application instance
}
 
module.exports = createApplication; // Export the factory function

In this flow:

• createApplication (i.e., express()) is a factory function.
• It internally uses the Application constructor (with new) to create an instance.
• It returns the Application instance to the user.

Thus, express() acts as a wrapper around the internal Application constructor, providing a clean, new-free API for creating app instances.

What Does express() Return? The Express Application Object#

When you call express(), it returns an Express application object (often called app). This object is the heart of your Express application, providing methods to define routes, middleware, and server behavior.

3.1 The Role of the app Object#

The app object is an instance of Express’s internal Application class. It encapsulates all the functionality needed to build a web server, including:

• Routing (e.g., app.get('/home', handler)).
• Middleware management (e.g., app.use(middleware)).
• Server configuration (e.g., app.set('port', 3000)).
• Starting the server (e.g., app.listen(3000)).

3.2 Key Methods of the app Object#

Here’s a quick overview of essential app methods:

Example: Using the app Object#

A minimal Express app demonstrates how express() and the app object work together:

const express = require('express');
const app = express(); // Call express() to get the app object
 
// Define a route
app.get('/', (req, res) => {
  res.send('Hello, Express!');
});
 
// Start the server
app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

In this example:

• express() creates the app instance.
• app.get() defines a route for the root URL.
• app.listen() starts the server on port 3000.

Common Misconceptions#

Let’s address a few myths about express():

• Myth 1: express is a class.
  False. express is a function (the factory function), not a class. The app object is an instance of the internal Application class, but express itself is not a class.

• Myth 2: express() is a method of the Express module.
  False. The Express module exports a function directly, not an object with methods. express() is the exported function itself, not a method.

• Myth 3: You need new to call express().
  False. While new express() technically works (Express handles it gracefully), it’s unnecessary and not recommended. The factory function is designed to be called without new.

Conclusion#

express() is a factory function exported by the Express.js module. It creates and returns an Application instance (the app object) by internally using an Application constructor. It is neither a method (since it’s not attached to an object) nor a traditional constructor (since it’s called without new).

Understanding express() is key to mastering Express.js, as it’s the gateway to creating the application object that powers your server. By demystifying its origins and behavior, you’ll have a stronger foundation for building Express applications.

References#

• Express.js Official Documentation
• Express.js Source Code (GitHub)
• MDN Web Docs: Factory Functions
• MDN Web Docs: Constructors