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;
I'm working on my first NodeJS project. I started building modules in the classical way as I read on books and over the internet.
As the project started growing I decided to split modules in small reusable pieces. That lead me to have a lot of require at the top of the file and sometime the risk to tackle circular dependencies. Moreover, this approach, doesn't really fits with testing because I had to require all the dependencies to make tests. I asked other developers a better way to solve this problem and most of them suggested me to use dependency injection with function constructor.
Suppose I have ModuleA and ModuleB,
ModuleC requires both ModuleA and ModuleB. Instead of requiring these modules an the top of the page I should pass them as argument in a constructor function.
e.g.
module.exports = function ModuleC(moduleA, moduleB) {
//module implementation here....
function doSomething() {}
return {
doSomething
}
}
the problem with this approach, that at first looked good, is that in the main entry point of the application I have to require and instantiate all the module to pass.
const ModuleA = require("./moduleA");
const ModuleB = require("./moduleB");
const ModuleC = require("./moduleC");
const moduleA = new ModuleA();
const moduleB = new ModuleB();
const moduleC = new ModuleC(moduleA, moduleB);
moduleC.doSomething();
Now with only 3 modules I had to write 7 line of code to use a function of a module. If I had 20 modules to work with the main entry point of the application would be a nightmare.
I guess this is not the best approach and even with this method, testing is not easy.
So, I ask you to suggest/explain me the best way to achieve this simple task that I'm finding, maybe harder than it is, while starting exploring the NodeJS word. Thanks.
Code re-usability can also be achieved if you put all of your codes in a single file. Creating smaller modules is not the only solution. consider the following codes written in a file allLogic.js(let).
var allFunctions = function(){ };
var allFunctionsProto = allFunctions.prototype;
allFunctionsProto.getJSONFile = function(fileName){
var that = this; //use 'that' instead of 'this'
//code to fetch json file
};
allFunctionsProto.calculateInterest = function(price){
var that = this; //use 'that' instead of 'this'
//code to calculate interest
};
allFunctionsProto.sendResponse = function(data){
var that = this; //use 'that' instead of 'this'
//code to send response
};
//similary write all of your functions here using "allFunctionsProto.<yourFunctionName>"
module.exports = new allFunctions();
Whenever I need to get any JSON file I know that the logic to get JSON file has already been written in allLogic.js file, hence I will require this module and use that as below.
var allLogic = require('../path-to-allLogic/allLogic.js');
allLogic.getJSON();
this approach is far better than creating tons of module for each work. Of course if the module gets longer you can create new module but in that case you need to consider separation of concern otherwise the circular dependency will haunt you.
As you are using you moduleA and moduleB in moduleC, if you put all of the codes from moduleA, moduleB and moduleC in a single module as I have pointed out you can reference the functions and all of the separate functions inside that module using that and those are also available after require.
I have a variable in my server.js, lets call it 'a'. When I require a module, it doesn't have access to the variable a. For example:
server.js
myModule = require('./myModule.js');
var a = 'Hello!'
myModule.say();
myModule.js
exports.say = function () {
console.log(a);
}
How can I make it so myModule.js can access the variables in server.js without having function arguments?
server.js:
myModule = require('./myModule.js');
global.a = 'Hello!'
myModule.say();
myModule.js:
exports.say = function () {
console.log(global.a);
}
However, please keep in mind globals are usually discouraged in Node.js (and JavaScript in general). Isn't the point of a module to encapsulate certain functionality? If so, it shouldn't depend on outside variables existing or being defined.
Ideally, you want to pass in the required information into the module via some sort of initialization function or configuration parameters.
I want to use one module feature within another module
file main.js
var _ = require("./underscore.js");
var foo = require("./bar.js");
foo.publish(...);
file bar.js
(function(e) {
var array = [...];
e.publish = function(t, args) {
_.each(array, function(...) {...});
});
})(exports);
I've tried a couple of variations, but I am not sure of the best way around this error:
ReferenceError: _ is not defined
Yes, you should use in every module which needs that variable, for the case of you example.
var _ = require("./underscore.js");
But if you need to transfer one object instance between several modules you can use process object to make it global. But that is BAD practice.
process._ = require("./underscore.js");
Good way to pass object instances between the modules is to pass them as function arguments, but you need to change you bar.js to return a factory function, not an object itself.
module.exports = function(_) {
var e = {};
var array = [...];
e.publish = function(t, args) {
_.each(array, function(...) {...});
});
return e;
}
In main.js:
var _ = require("./underscore.js");
var foo = require("./bar.js")(_);
foo.publish(...);
I hope you got the point.
The variable to which you assign the result is local to the module main.js, so you can't access it in bar.js. Instead, you should require the underscore module in bar.js as well.
You could also skip the var when declaring _ and make it a global variable, but that brings all the problems of global variables, like:
the bar.js module is not explicit about its dependencies, so it's harder to understand that it expects underscore to be required globally;
it requires a specific initialization order - if you later move the _ = require(underscore), you'll get undefined errors again, and it might be hard to understand why;
every other module that requires bar.js needs to also require underscore.
I don't think I quite understand how exports work in Node.js. In a bit of sample code, I noticed the exports object used in this manner:
exports = mongoose = require('mongoose')
mongoose.connect(config.db.uri)
exports = Schema = mongoose.Schema
What is happening behind the scenes when you use exports = twice like that? Looks to me like "mongoose" should not be exported. I did this quick test:
var foo
, bar
exports = foo = 'foo'
exports = bar = 'bar'
// reports 'bar' only
console.log(exports)
and the second test does overwrite the first export.
My guess is the original author of that sample code is confused about module.exports vs exports. To use the exports object, you must add properties to it like this:
exports.size = 42;
If you re-assign the exports variable to a new object, you basically lose access to the global exports object that node.js provided for you. If you do this twice or three or N times, the effect is the same. It's useless. For example: mod_b.js
var realExports = exports;
realExports.height = 42;
var exports = {};
exports.weight = 43;
And in mod_a.js
var mod_b = require('./mod_b');
console.dir(mod_b);
Run node mod_a.js and you get:
{ height: 42 }
Notice height is there but weight is not. Now, what you can do is assign module.exports to be an object, and that is the object that will be returned when another module requires your module. So you will see things like.
var x = 10;
var y = 20;
module.exports = {x: x, y: y};
Which will do what you expect. Here's some informative articles on the details.
Node.js Module – exports vs module.exports
What is the purpose of NodeJS module.exports and how do you use it?
Mastering Node
As an extension to #Peter-Lyons answer, your example is valid, your test case is incorrect.
Exporting variables like this is not recommended, but by doing that, you are exposing this variables into a global namespace.
e.g. test0.js
'use strict';
var assert = require('assert');
var test1 = require('./test1');
assert(globalVar1 === 123);
assert(globalVar2.a === 123);
test1.js -- mind the missing 'use strict';
exports = globalVar1 = 123;
exports = globalVar2 = {a:123};
Once again, it's valid, but not recommended.
Cheers.