context:
object named multiVerse that I need to access in another file that isn't server.js (main file)
using module.exports={multiVerse} in server.js
requiring the main file using const {multiVerse} = require('./server.js');
when using function to arrange object (made in the same file I require main, calling its class as module on server.js) it tells me multiVerse is undefined
when requiring the class into server.js to use arrangeBodies() I am doing const NBody = require('./nbody') const nbody = new NBody(); before the multiVerse object is initialized
questions:
Am I exporting it correctly? Why am I getting undefined?
Wondering what would be the best practice for my case.
I have a variable I need to set its value on app load and access this value many times across my code. what is the best way to get this value?
Right now I'm just overriding a config file property. Does a global variable is better? Is there another way to do this?
The priority standard for configs IMHO is:
command line parameter
environment var
local config file
global config file
if no cli parameter found, fall to look into environment vars, then local config file, then global
I do this.
Put the variable in .env file:
# .env
APP_PORT=4000
In my app source code, I create a file named constants.js:
// constants.js
import { load as loadEnvVars } from 'dotenv'; // run `npm i dotenv` to install
// load the .env file content to process.env
loadEnvVars();
// export the variables I need
export const APP_PORT = process.env.APP_PORT || 3000;
I import that file when I need it like this:
// server.js
import Express from 'express';
// import the constant
import { APP_PORT } from './constants';
const app = Express();
app.listen(APP_PORT, () => console.log('server deployed');
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)
I am Unable to import app.js in other modules in expressJs. It's importing but i am unable to use functions defined in app.js file
Code i have is this-
I have this
app.js
var app = express();
var expressWs = require('express-ws')(app);
.
.
.
wss= expressWs.getWss();
// console.log(wss); // works fine
app.getWss=function(){
return "hello";
};
app.listen(3001);
module.exports = app;
in a file inside /socketroutes/socketroutes.js
var app = require('../app');
console.log(app); // prints an empty object {}
console.log(app.getWss()) // returns undefined function and doesn't work
I want to use some variables or functions defined in app.js in another modules. because instance of websocket server should be created only once. i cannot create instance of wss in every module.
It's mostly because of a circular dependency.
Try moving the module.exports line to the beginning of the app.js file.
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);