Initializing function handler for Java Script modules

Question

Initializing function handler for Java Script modules

As I was diving into the socket.io's get-started guide, I stumbled upon a module requirement that left me puzzled:

var app = require('express')();
var http = require('http').Server(app);

The explanation provided by the author is as follows:

Express initializes app to be a function handler that you can supply to an HTTP server (as seen in line 2)

I couldn't find any clear explanations for this specific usage in the documentation I checked. So, what exactly is going on here? Although Socket.io is mentioned, my question is unrelated to it and is purely focused on understanding this coding snippet. Not sure if this falls under a simple or complex query...

javascript express socket.io handler require

Answer 1

Answer №1

Importing express means bringing in something that the Express framework decides to make available through export. The code snippet in index.js can be found here:

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

This shows that it imports from a subdirectory within the project, specifically importing express.js. By default (as seen here), it exports a function named createApplication:

exports = module.exports = createApplication;

/**
 * Create an express application.
 *
 * @return {Function}
 * @api public
 */

function createApplication() {
    // ...
}

When you require('express'), it points to function createApplication(), which is simply a function. This explains why there are parentheses like (); they execute/call the returned function.

Explore more about Higher-order functions.

Answer 2

Importing express means bringing in something that the Express framework decides to make available through export. The code snippet in index.js can be found here:

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

This shows that it imports from a subdirectory within the project, specifically importing express.js. By default (as seen here), it exports a function named createApplication:

exports = module.exports = createApplication;

/**
 * Create an express application.
 *
 * @return {Function}
 * @api public
 */

function createApplication() {
    // ...
}

When you require('express'), it points to function createApplication(), which is simply a function. This explains why there are parentheses like (); they execute/call the returned function.

Explore more about Higher-order functions.

Initializing function handler for Java Script modules

Answer №1

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