I have a app.js
const express = require('express');
const app = express();
const server = require('./server.js');
// app.use
const io = require('socket.io').listen(server);
io.on('connection', function (socket) {
...
});
module.exports = app;
And a server.js
const app = require('./app');
const server = app.listen(5000 || process.env.PORT, () => {
console.log('App listening on port 5000!');
})
module.exports = server;
If I put the server in a separated file the socket is not working, but if I start the server inside the app.js the socket works.
What I'm doing wrong?
The issue here is that you have a circular dependency where app.js is loading server.js and server.js is loading app.js. You can't do that for this type of code.
It has an issue because you're trying to load server.js from within app.js and then in the process of loading server.js, it attempts to load app.js and get its exports, but app.js hasn't finished loading yet and thus hasn't even returned its exports yet. So, the loader either thinks there are no exports or recognizes the circular request (I'm not sure which), but in either case the exports from app.js don't work because of the circular requires.
There are several different ways to solve this. The two most common ways are:
Break some code into a common third module that each of these can load and only have one of these load the other.
Rather than having server.js load app to get the app object, have app.js pass the app object to server.js in a constructor function rather than trying to execute at module load time.
Here's how the constructor function idea would work:
app.js
const express = require('express');
const app = express();
// load server.js and call it's constructor, passing the app object
// that module constructor function will return the server object
const server = require('./server.js')(app);
// app.use
const io = require('socket.io').listen(server);
io.on('connection', function (socket) {
...
});
module.exports = app;
server.js
// export constructor function that must be called to initialize this module
module.exports = function(app) {
const server = app.listen(5000 || process.env.PORT, () => {
console.log('App listening on port 5000!');
});
return server;
};
So, rather than server.js trying to load the app.js module to get the app object, the app object is "pushed" to it with a constructor function. This prevents the circular dependency.
I have two js files in Electron (which uses Nodejs) and I try to export from one and require in another.
app.js:
App = {
server: {
host: '192.168.0.5',
user: 'root',
}
ping: function() {
}
}
exports.App = App
I have tried every way possible of exporting, including module.exports = App, module.exports.App = App and so on.
ping.js first attempt:
var App = require('../app.js') // I have also tried adding .App to the end
console.log(App) // This returns an object which contains the App object
ping.js second attempt:
var App = require('../app.js')
App.x = 'y'
console.log(App) // this returns an object which contains the App object and the x property
It may appear that App contains another App object, but console.log(App.App) says it doesn't exist.
The first thing I'd to do solve this would be to make sure I'm using the full path to the required module, as in:
const Path = require('path')
const App = require(Path.join(__dirname,'../app')) // the .js isn't needed here.
Note that this assumes that the app.js file is in the immediate parent directory of the one in which the application runs.
If that doesn't work, I'd make sure the files are where you think they are, and that the process you're running is located within the file system where you think it is. You can determine this by adding this to the top of your main script file:
console.log("current working directory:",process.cwd())
Or in es6:
console.log(`current working directory: %s`, process.cwd())
If the printed directory doesn't match your assumptions, modify your require statement accordingly.
And for the record, the "correct" way to export your App map would be to:
const App = {
...
}
module.exports = App
Or using es7:
export default App = {
...
}
(See export for more on es7 modules.)
Either way, you'd then require the module as:
const App = require(PATH_TO_APP)
In Node.js I'm using Socker.io in my main.js like this
const io = require('socket.io')(http);
Also Im using a "sub"-file like api.js which I want to use to delecate some of my business logic away from the main. So I imported also this one like
const api = require('./api.js');
In my api.js how can I now use the socket.io framework? Can I access the instance from above from a different file? Or do I have to pass the "io"-object like this: api.myFoo(io);
Every place that you say require('module') you will get the same instance of that module.
But here when you want to share the return value of a function, then you have to export it explicitly:
const io = require('socket.io')(http);
module.exports = io;
in some module, and require it in another modules by:
const io = require('./your-module');
Other option would be to pass it as an argument to other modules, like this:
const io = require('socket.io')(http);
const api = require('./api.js')(io);
but in that case your api.js would have to export a function that takes io as an argument:
module.exports = (io) => {
return ... // return whatever was exported before
};
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:
if a file called some_module.js exists in the current folder node will load that, otherwise:
Node looks in the current folder for a node_modules folder with a some_module folder in it.
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!
let me answer this question by an example.
create 2 javascript files.
play1.js and express.js
//express.js
function createApplication(){
var app = 'app';
return app;
}
module.exports = createApplication;
//keep in mind that we are not doing module.exports = {createApplication}
now import express.js in play1.js file
//play1.js
var express = require('./express);
var app = express();
// this will call createApplication function as app is referencing to it.
console.log(app); // "app"
Whenever you import a module like
const express = require('express')
express is a module with functions or objects or variables assigned to it .
take a look at /lib/express
you are able to access the function createApplication inside express module as express() because the function is assigned directly to the module like
exports = module.exports = createApplication;
function createApplication(){
var app = function(req, res, next) {
app.handle(req, res, next);
};
//other codes
}
so you are able to access the function createApplication just calling express() as function
now when you check out the other section of the express library, you can see a bunch of other objects attached to the exports special object as well.
/**
* Expose the prototypes.
*/
exports.application = proto;
exports.request = req;
exports.response = res;
/**
* Expose constructors.
*/
exports.Route = Route;
exports.Router = Router;
// other exports
these objects or function assigned to export special object can be accessed from the import section using express as an object.
express.{name}
express.Route
express.Router etc
In the end you are just exporting a bunch of methods or objects that are attached to the module.export special object inside express js file
to read more on module.export special object go here
1- var express = require('express');
first line require the express package .js file, und "require" it's only returnd what was exported in the js file with (module.exports).
so we have only pointer to this function .
2- var app = express();
in second line, we use 'app' as pleaceholder to receive the output from express() function, which is an object, we can use it in our code (by accessing his methods and properties like any other Class )
in other words, we use the 'app' Object, which which produced from 'express()' function, that we imported from 'express.js' file .
NOTE 1) and of course we can give any name instead of 'app' , but it's a good practice when you follow what the most developers use to name this packages, that make easier to understand your code specialty when you work in team.
NOTE 2) after ES6, we use 'const' instead of 'var' .
Simple what we wrote in node js when we require a modules for our application
const modue_need1=require('module_name');
const modue_need2=require('module_name2');
const modue_need3=require('module_name3');
const modue_need4=require('module_name4');
const modue_need....=require('module_name.....');
So for every module, we need to write such a big code, and time-consuming now to reduce these lengthy codes and time slice what we do? We need node js Framework like Express js
which will overcome these problems mean "write less, do more"
we just use this two-line and all the requirement(modules) about our app will be their in-app object which we can use whenever we need so do not need to call require for every module.
"write less, do more"
const express=require('express');
const app=express();
console.log(app);
I have an Express.js web app. In the main app.js file, I require() a bunch of third party dependencies. Recently I've started extracting parts of my app.js code into separate modules, e.g.
// in app.js
var users = require('./modules/users');
// and then e.g.
// users.add(doc);
Inside my modules, I sometimes need to use objects that are available in my main app.js file and I'm wondering what's the best way to pass references to those object to my modules.
My current approach:
// in app.js
// pass needed object references in initialization step
users.init([db, logger]);
and
// in modules/users.js
var db, logger;
exports.init = function (deps) {
db = deps[0];
logger = deps[1];
};
Does this approach make sense? Is there a better way to perform this?
Sure, just use modules! :)
// db.js
// create db instance here rather than in app.js
module.exports = db;
And
// logger.js
// create logger instance here rather than in app.js
module.exports = logger;
Then
// app.js
var db = require('./db');
And
// lib/somewhere.js
var db = require('../db');
This way you're able to rely on the CommonJS dependency injection system rather than on doing the dependency injection all by yourself (passing references around instead of requireing a module into yours).
The reason why this works as expected is that modules are only interpreted once, so if you instantiate everything once as opposed to using a factory function, it just works.
You should just be able to require modules as normal:
// users.js
var db = require('./db');
exports.init = function() {
// use db in here
};
However, sometimes this isn't possible and you will need to explicitly pass in the module.
One way to do it is to pass in dependencies when you require the module:
// users.js
module.exports = function(db, logger) {
return {
init: function() { /* use db and logger in here */}
};
}
// app.js
var db = ...;
var logger = ...;
var users = require('./users')(db, logger);
users.init();
This is the pattern that I personally prefer, I think it's cleaner to pass dependencies into the require than into some init method like you have in your example.
You'll see this done in ExpressJS code quite a lot, for example when we have all our routes in another file and need to pass our app instance around:
require('./routes')(app);
If you need something to be initialized specifically in app.js rather than their own module you can export them from app.js and then require app.js:
// app.js
var db = require("db"),
logger = require("logger");
// do your initialization with db and logger
module.exports = { db: db, logger: logger };
and then:
// users.js
var db = require("./app").db,
logger = require("./app").logger;
// use db and logger
This works because as #Nico mentioned, modules are interpreted once and app.js will not be interpreted each time it is required from elsewhere.