I have tons of files where I have to add this function.
const Media = require("../../functions/Media");
Media(....);
Is there any way to:
Directly add Media() function without requiring the file
Try declaring it as global in index file:
const {Media} = require('./functions/Media');
global.MediaGlobal = function Media(input) {
Media(input)
}
It really depends on the framework you are using. You need to find the starting index file and append all global functions there. Samples are given below.
// Override Globals in Backend Nodejs
if (typeof global !== "undefined") {
global.someGlobalFunction = function () {};
}
// Override Globals in Frontend Nodejs/reactjs
// Override Globals in Frontend javascript written on nodejs
if (typeof window !== "undefined") {
window.someGlobalFunction = function () {};
}
Node.js + Express (Backend)
Look for file define server, app.listen, Defined on top before import other modules
Node.js + React (Frontend)
Look for file define react.render, app.jsx, Defined on top before import other modules
Learn the concept of polyfill:
Best way to polyfill ES6 features in React app that uses create-react-app
I have a Node REST API using Express and JavaScript. I created a Database class that is able to connect, disconnect and execute the queries. My app.js (the entry file) creates an instance database of this and launches the express server. On requests I have the following flow:
requests call the middleware functions and finally the controller
the controller calls some other classes and passes in the data
Lastly these classes call the query files to access the database
The query files themselves need to access the instance of the database class. They can call the query function and pass in their prepared statement. I am not sure how to transport the instance database from my app.js (entry file) all the way to those query files.
I found multiple solutions:
using global variables
add the instance to the request object
add the instance to the response locals
add the instance to the app locals
What is the best way/practice to transport variables from one file to the whole application?
Use service architecture or dependency injection.
Service architecture Way:
Make a directory structure like this:
root (directory)
|
|-->app.js
|-->controllers (directory)
|-->services (directory)
|
|-> DatabaseService.js
|-> XYZService.js
|-> index.js
Now in your index.js file require the service classes and export the instances of those classes, like so:
var Database = require('./DatabaseService')
var XYZ = require('./XYZService')
module.exports = {
database: new Database(),
xyz: new XYZ()
}
Now, require these services wherever you want in your code, like so:
// SomeController.js
var database = require('../services').database
...
// DO WHATEVER YOU WANT WITH 'database' variable
...
Dependency Injection Way:
I'm assuming you are using express routes. Do a dependency injection by wrapping your route-controller inside a lambda function, like so:
api.get('/user/:id', function (req, res) { // <-- wrapping
var database = new Database()
return yourRouteController(req, res, database) // <-- Inject your database instance here
})
PS I personally prefer the Service way as it's more modular and readable.
The only other solution I've seen for something like this is using dependency injection. It's still using the global variable but instead of tossing it down the line from one class to the other, you could call up that particular instance of your db connection at any point while your app is runnning.
Let's say I have an application as following:
server.js (main) is requiring different external node packages, like underscore.js.
var Underscore = require("underscore");
server.js is also requiring some modules defined in my application it-self; For example it could requires a Router module, to handle Express routes.
var Router = require("./sources/router.js");
I have then my router.js file as following :
var Router;
Router = (function() {
function Router(app, oauth) {
app.get('/', function(request, response) {
// ...
});
}
return Router;
})();
module.exports = Router;
Questions:
If I want to use underscore inside my Router module, should I re-require again ? Should I do that for every modules ? What is the impact?
I would end up with something like:
var Router;
Router = (function() {
Router.prototype.underscore = require("underscore");
function Router(app, oauth) {
app.get('/', function(request, response) {
// this.underscore. ...
// using underscore ...
});
}
return Router;
})();
module.exports = Router;
and
var Underscore = require("underscore");
var Router = require("./sources/router.js");
router = new Router();
I could obviously also inject as a parameter it when initializing Router, but this doesn't look to me like a viable option in an application where I may end up using dozens of packages, especially for very general purpose package like this one.
var underscore = require("underscore");
var Router = require("./sources/router.js");
var router = new Router(underscore);
Alternatively I could set the underscore var as a global one, but I don't really like this option.
Is there any other options ?
What is the impact of systematically importing packages in every modules - in term of execution time, memory ?
I would like to understand the behavior of the node engine in such cases.
Yes, you should just require it again. Node caches required modules, so the second time you require something it doesn't actually run that file; it just returns the cached object. So the memory impact is basically 0 (an extra pointer to the same object, more or less) and the execution time is similarly negligible (the cost of a lookup in an object by the module name).
This means that the objects returned by the two requires aren't just identical; they're literally the same object. Any change to one will affect the other. Because of this, you can extend the module in one place and get those extensions everywhere.
What is the purpose of Node.js module.exports and how do you use it?
I can't seem to find any information on this, but it appears to be a rather important part of Node.js as I often see it in source code.
According to the Node.js documentation:
module
A reference to the current
module. In particular module.exports
is the same as the exports object. See
src/node.js for more information.
But this doesn't really help.
What exactly does module.exports do, and what would a simple example be?
module.exports is the object that's actually returned as the result of a require call.
The exports variable is initially set to that same object (i.e. it's a shorthand "alias"), so in the module code you would usually write something like this:
let myFunc1 = function() { ... };
let myFunc2 = function() { ... };
exports.myFunc1 = myFunc1;
exports.myFunc2 = myFunc2;
to export (or "expose") the internally scoped functions myFunc1 and myFunc2.
And in the calling code you would use:
const m = require('./mymodule');
m.myFunc1();
where the last line shows how the result of require is (usually) just a plain object whose properties may be accessed.
NB: if you overwrite exports then it will no longer refer to module.exports. So if you wish to assign a new object (or a function reference) to exports then you should also assign that new object to module.exports
It's worth noting that the name added to the exports object does not have to be the same as the module's internally scoped name for the value that you're adding, so you could have:
let myVeryLongInternalName = function() { ... };
exports.shortName = myVeryLongInternalName;
// add other objects, functions, as required
followed by:
const m = require('./mymodule');
m.shortName(); // invokes module.myVeryLongInternalName
This has already been answered but I wanted to add some clarification...
You can use both exports and module.exports to import code into your application like this:
var mycode = require('./path/to/mycode');
The basic use case you'll see (e.g. in ExpressJS example code) is that you set properties on the exports object in a .js file that you then import using require()
So in a simple counting example, you could have:
(counter.js):
var count = 1;
exports.increment = function() {
count++;
};
exports.getCount = function() {
return count;
};
... then in your application (web.js, or really any other .js file):
var counting = require('./counter.js');
console.log(counting.getCount()); // 1
counting.increment();
console.log(counting.getCount()); // 2
In simple terms, you can think of required files as functions that return a single object, and you can add properties (strings, numbers, arrays, functions, anything) to the object that's returned by setting them on exports.
Sometimes you'll want the object returned from a require() call to be a function you can call, rather than just an object with properties. In that case you need to also set module.exports, like this:
(sayhello.js):
module.exports = exports = function() {
console.log("Hello World!");
};
(app.js):
var sayHello = require('./sayhello.js');
sayHello(); // "Hello World!"
The difference between exports and module.exports is explained better in this answer here.
Note that the NodeJS module mechanism is based on CommonJS modules which are supported in many other implementations like RequireJS, but also SproutCore, CouchDB, Wakanda, OrientDB, ArangoDB, RingoJS, TeaJS, SilkJS, curl.js, or even Adobe Photoshop (via PSLib).
You can find the full list of known implementations here.
Unless your module use node specific features or module, I highly encourage you then using exports instead of module.exports which is not part of the CommonJS standard, and then mostly not supported by other implementations.
Another NodeJS specific feature is when you assign a reference to a new object to exports instead of just adding properties and methods to it like in the last example provided by Jed Watson in this thread. I would personally discourage this practice as this breaks the circular reference support of the CommonJS modules mechanism. It is then not supported by all implementations and Jed example should then be written this way (or a similar one) to provide a more universal module:
(sayhello.js):
exports.run = function() {
console.log("Hello World!");
}
(app.js):
var sayHello = require('./sayhello');
sayHello.run(); // "Hello World!"
Or using ES6 features
(sayhello.js):
Object.assign(exports, {
// Put all your public API here
sayhello() {
console.log("Hello World!");
}
});
(app.js):
const { sayHello } = require('./sayhello');
sayHello(); // "Hello World!"
PS: It looks like Appcelerator also implements CommonJS modules, but without the circular reference support (see: Appcelerator and CommonJS modules (caching and circular references))
Some few things you must take care if you assign a reference to a new object to exports and /or modules.exports:
1. All properties/methods previously attached to the original exports or module.exports are of course lost because the exported object will now reference another new one
This one is obvious, but if you add an exported method at the beginning of an existing module, be sure the native exported object is not referencing another object at the end
exports.method1 = function () {}; // exposed to the original exported object
exports.method2 = function () {}; // exposed to the original exported object
module.exports.method3 = function () {}; // exposed with method1 & method2
var otherAPI = {
// some properties and/or methods
}
exports = otherAPI; // replace the original API (works also with module.exports)
2. In case one of exports or module.exports reference a new value, they don't reference to the same object any more
exports = function AConstructor() {}; // override the original exported object
exports.method2 = function () {}; // exposed to the new exported object
// method added to the original exports object which not exposed any more
module.exports.method3 = function () {};
3. Tricky consequence. If you change the reference to both exports and module.exports, hard to say which API is exposed (it looks like module.exports wins)
// override the original exported object
module.exports = function AConstructor() {};
// try to override the original exported object
// but module.exports will be exposed instead
exports = function AnotherConstructor() {};
the module.exports property or the exports object allows a module to select what should be shared with the application
I have a video on module_export available here
When dividing your program code over multiple files, module.exports is used to publish variables and functions to the consumer of a module. The require() call in your source file is replaced with corresponding module.exports loaded from the module.
Remember when writing modules
Module loads are cached, only initial call evaluates JavaScript.
It's possible to use local variables and functions inside a module, not everything needs to be exported.
The module.exports object is also available as exports shorthand. But when returning a sole function, always use module.exports.
According to: "Modules Part 2 - Writing modules".
the refer link is like this:
exports = module.exports = function(){
//....
}
the properties of exports or module.exports ,such as functions or variables , will be exposed outside
there is something you must pay more attention : don't override exports .
why ?
because exports just the reference of module.exports , you can add the properties onto the exports ,but if you override the exports , the reference link will be broken .
good example :
exports.name = 'william';
exports.getName = function(){
console.log(this.name);
}
bad example :
exports = 'william';
exports = function(){
//...
}
If you just want to exposed only one function or variable , like this:
// test.js
var name = 'william';
module.exports = function(){
console.log(name);
}
// index.js
var test = require('./test');
test();
this module only exposed one function and the property of name is private for the outside .
There are some default or existing modules in node.js when you download and install node.js like http, sys etc.
Since they are already in node.js, when we want to use these modules we basically do like import modules, but why? because they are already present in the node.js. Importing is like taking them from node.js and putting them into your program. And then using them.
Whereas Exports is exactly the opposite, you are creating the module you want, let's say the module addition.js and putting that module into the node.js, you do it by exporting it.
Before I write anything here, remember, module.exports.additionTwo is same as exports.additionTwo
Huh, so that's the reason, we do like
exports.additionTwo = function(x)
{return x+2;};
Be careful with the path
Lets say you have created an addition.js module,
exports.additionTwo = function(x){
return x + 2;
};
When you run this on your NODE.JS command prompt:
node
var run = require('addition.js');
This will error out saying
Error: Cannot find module addition.js
This is because the node.js process is unable the addition.js since we didn't mention the path. So, we have can set the path by using NODE_PATH
set NODE_PATH = path/to/your/additon.js
Now, this should run successfully without any errors!!
One more thing, you can also run the addition.js file by not setting the NODE_PATH, back to your nodejs command prompt:
node
var run = require('./addition.js');
Since we are providing the path here by saying it's in the current directory ./ this should also run successfully.
A module encapsulates related code into a single unit of code. When creating a module, this can be interpreted as moving all related functions into a file.
Suppose there is a file Hello.js which include two functions
sayHelloInEnglish = function() {
return "Hello";
};
sayHelloInSpanish = function() {
return "Hola";
};
We write a function only when utility of the code is more than one call.
Suppose we want to increase utility of the function to a different file say World.js,in this case exporting a file comes into picture which can be obtained by module.exports.
You can just export both the function by the code given below
var anyVariable={
sayHelloInEnglish = function() {
return "Hello";
};
sayHelloInSpanish = function() {
return "Hola";
};
}
module.export=anyVariable;
Now you just need to require the file name into World.js inorder to use those functions
var world= require("./hello.js");
The intent is:
Modular programming is a software design technique that emphasizes
separating the functionality of a program into independent,
interchangeable modules, such that each contains everything necessary
to execute only one aspect of the desired functionality.
Wikipedia
I imagine it becomes difficult to write a large programs without modular / reusable code. In nodejs we can create modular programs utilising module.exports defining what we expose and compose our program with require.
Try this example:
fileLog.js
function log(string) { require('fs').appendFileSync('log.txt',string); }
module.exports = log;
stdoutLog.js
function log(string) { console.log(string); }
module.exports = log;
program.js
const log = require('./stdoutLog.js')
log('hello world!');
execute
$ node program.js
hello world!
Now try swapping ./stdoutLog.js for ./fileLog.js.
What is the purpose of a module system?
It accomplishes the following things:
Keeps our files from bloating to really big sizes. Having files with e.g. 5000 lines of code in it are usually real hard to deal with during development.
Enforces separation of concerns. Having our code split up into multiple files allows us to have appropriate file names for every file. This way we can easily identify what every module does and where to find it (assuming we made a logical directory structure which is still your responsibility).
Having modules makes it easier to find certain parts of code which makes our code more maintainable.
How does it work?
NodejS uses the CommomJS module system which works in the following manner:
If a file wants to export something it has to declare it using module.export syntax
If a file wants to import something it has to declare it using require('file') syntax
Example:
test1.js
const test2 = require('./test2'); // returns the module.exports object of a file
test2.Func1(); // logs func1
test2.Func2(); // logs func2
test2.js
module.exports.Func1 = () => {console.log('func1')};
exports.Func2 = () => {console.log('func2')};
Other useful things to know:
Modules are getting cached. When you are loading the same module in 2 different files the module only has to be loaded once. The second time a require() is called on the same module the is pulled from the cache.
Modules are loaded in synchronous. This behavior is required, if it was asynchronous we couldn't access the object retrieved from require() right away.
ECMAScript modules - 2022
From Node 14.0 ECMAScript modules are no longer experimental and you can use them instead of classic Node's CommonJS modules.
ECMAScript modules are the official standard format to package JavaScript code for reuse. Modules are defined using a variety of import and export statements.
You can define an ES module that exports a function:
// my-fun.mjs
function myFun(num) {
// do something
}
export { myFun };
Then, you can import the exported function from my-fun.mjs:
// app.mjs
import { myFun } from './my-fun.mjs';
myFun();
.mjs is the default extension for Node.js ECMAScript modules.
But you can configure the default modules extension to lookup when resolving modules using the package.json "type" field, or the --input-type flag in the CLI.
Recent versions of Node.js fully supports both ECMAScript and CommonJS modules. Moreover, it provides interoperability between them.
module.exports
ECMAScript and CommonJS modules have many differences but the most relevant difference - to this question - is that there are no more requires, no more exports, no more module.exports
In most cases, the ES module import can be used to load CommonJS modules.
If needed, a require function can be constructed within an ES module using module.createRequire().
ECMAScript modules releases history
Release
Changes
v15.3.0, v14.17.0, v12.22.0
Stabilized modules implementation
v14.13.0, v12.20.0
Support for detection of CommonJS named exports
v14.0.0, v13.14.0, v12.20.0
Remove experimental modules warning
v13.2.0, v12.17.0
Loading ECMAScript modules no longer requires a command-line flag
v12.0.0
Add support for ES modules using .js file extension via package.json "type" field
v8.5.0
Added initial ES modules implementation
You can find all the changelogs in Node.js repository
let test = function() {
return "Hello world"
};
exports.test = test;
my routes are defined in an external folder
./routes
here's the way i define the routes in my server.js file
app.get('/', routes.index);
app.post('/validation', register.valid);
the register.valid module, which is originally written in
./routes/validation.js
is responsible for creating a new user account and register it into a database (MongoDB).
How can i access an object from server.js in validation.js ? First, i thought declaring the object before defining my routes would resolve the case, but actually it doesn't seem to be the solution.
I'm assuming your current code already does work (that is, you receive the posted data), but you need access to another object from validation.js.
If your code works, then you probably have this line in server.js:
var register = require('./routes/validation');
And you need acess to the variable obj in the validation module. You could have a function inside the validation module:
var foo;
exports.configure = function(obj) {
foo = obj;
}
The exports mean the variable configure will be accessible to modules which "require" the validation module. This way you can do, inside the server.js module:
register.configure(obj);
app.post('/validation', register.valid);
The exact configuration of this will depend on what you are actually trying to accomplish. Sometimes, for example, it's good to have a database object stored in a global variable.
Generally in this kind of structure server.js will create the app object and then pass that to individual routes modules via a function. I do it by having each router module export a single function like this:
//routes/validation.js
function setup(app) {
app.get(....blah
app.post(....blah
}
module.exports = setup;
Then I tie that together in server.js like this:
//server.js
var express = require('express');
var app = express();
require('./routes/validation')(app);
See also my express_code_structure sample project for other code organization tips.