当我需要“express”时，我需要知道它是一个对象还是一个函数

Question

Given my code

const express = require("express")
const app = express()


console.log(typeof app)
console.log(typeof express)

My terminal tells me that both of these variables are functions!

This makes no sense to me because I can access properties from each of them as if they were objects and yet when I do a typeof in console.log my terminal tells me they are both functions.

Can someone please explain to me what these variables are and why its possible to access their properties as if they were objects?

Answer 1

According to the docs , The express() function is a top-level function exported by the express module.

It is similar like having Class and Object . The variable express that you are declaring is acting as a class , while the variable app is the object of it which is used.

Answer 2

when I require “express” I need to know if its an object or function

It's both. Functions in Javascript are objects.

Without getting technical, express is a factory function that creates an app instance and app is an express instance. Now, technically, both express and app are functions. But, functions in Javascript are objects and can have properties. The express object has static properties. The app object behaves much like an instance of the express class. It has both methods and instance data.

Now, getting a little more technical...

const express = require('express') gets you a function and that function is also an object and has properties.

This specific function is a factory function that creates an app object when you call it that is also a function that has properties.

So, express also has properties that can be used such as:

express.static
express.json
express.urlencoded

And, it can be called as in:

const app = express();

Likewise, app is a function that has properties too. It can be used as a function as in:

const server = http.createServer(app);
server.listen(80);

Or, it can be used like an object as in:

const server = app.listen(80);

If you output this:

console.log(typeof express);
console.log(typeof app);

You will see this:

function
function

They are both functions. But, functions in Javascript are objects too and can have properties.

So, you can do things like this:

function myFunction() {
    return "ok";
}

myFunction.greeting = "Hello";

console.log(myFunction());                 // "ok"
console.log(myFunction.greeting);         // "Hello"

Looking further at both express and app , if you did this:

console.log(Object.getOwnPropertyNames(express));

You would get this:

[
  'length',         'name',
  'prototype',      'application',
  'request',        'response',
  'Route',          'Router',
  'json',           'query',
  'raw',            'static',
  'text',           'urlencoded',
  'bodyParser',     'compress',
  'cookieSession',  'session',
  'logger',         'cookieParser',
  'favicon',        'responseTime',
  'errorHandler',   'timeout',
  'methodOverride', 'vhost',
  'csrf',           'directory',
  'limit',          'multipart',
  'staticCache'
]

And, this:

console.log(Object.getOwnPropertyNames(app));

would get this:

[
  'length',          'name',            'prototype',
  'constructor',     '_events',         '_eventsCount',
  '_maxListeners',   'setMaxListeners', 'getMaxListeners',
  'emit',            'addListener',     'on',
  'prependListener', 'once',            'prependOnceListener',
  'removeListener',  'off',             'removeAllListeners',
  'listeners',       'rawListeners',    'listenerCount',
  'eventNames',      'init',            'defaultConfiguration',
  'lazyrouter',      'handle',          'use',
  'route',           'engine',          'param',
  'set',             'path',            'enabled',
  'disabled',        'enable',          'disable',
  'acl',             'bind',            'checkout',
  'connect',         'copy',            'delete',
  'get',             'head',            'link',
  'lock',            'm-search',        'merge',
  'mkactivity',      'mkcalendar',      'mkcol',
  'move',            'notify',          'options',
  'patch',           'post',            'propfind',
  'proppatch',       'purge',           'put',
  'rebind',          'report',          'search',
  'source',          'subscribe',       'trace',
  'unbind',          'unlink',          'unlock',
  'unsubscribe',     'all',             'del',
  'render',          'listen',          'request',
  'response',        'cache',           'engines',
  'settings',        'locals',          'mountpath',
  'router'
]

So, you can see they each have a lot of properties in addition to being functions.

---

Ok please tell me if I have got this correct. 1) When I do this… const express = require(“express”) I store a “Class” into the express variable. 2) Then when I do this… express.json() I am accessing the json() function inside the express class ?

As I said in my answer above that express variable represents a factory function. That's a function that, when called, creates an object for you. It's a different way of creating an object from directly calling a constructor as in new myObj() . express.json is a function that, when called, creates a middleware function for you that uses the parameters you passed the function.

The Express architecture differs a bit from a pure class style architecture. It uses a factory function that creates an instance (essentially of a class). And, then the app represents that instance, but also works as a request handler on its own.