I have two files basic.js basic1.js. Please find code below :
basic.js :
console.log("My first node statement");
const sum =(a,b)=> a+b;
console.log(sum(1,1));
module.exports.sum =sum;
basic1.js :
const sum1= require('./basic');
const EventE= require('events');
const eventEmitter = new EventE();
eventEmitter.on('firstEvent',(a,b)=> {
console.log(sum1.sum(a, b));
});
eventEmitter.emit('firstEvent',5,6);
on running basic1.js I am getting below output which seems incorrect as It should just called the function.
node basic1.js
My first node statement
2
11
As indicated in the comments, anytime you require a module, it runs all of the top level statements in that module. This is so that any dynamically created/assigned objects, functions, constants, etc actually exist. It's very similar to adding a script to your webpage in a browser. You can either remove the console statements in your required file, or move them into a scope that is not automatically run (a function expression or a method, for instance).
Additionally in the comments, it was requested to be able to simply use the function. The reason this is occuring is because of this line:
module.exports.sum =sum;
Basically you are returning an object with a property named sum. This is because, by default and for simplicity, module.exports is an object with none of its own keys. In code terms, the module results as:
{
sum: sum
}
To fix this, change the line to:
module.exports = sum;
Now, you are simply returning the function and only the function.
I have multiple different files broken out like so:
index.js
utils.js
ClassA/
index.js
base.js
utils contains a number of utility functions used everywhere. ClassA/index.js contains the constructor for ClassA as well as requires base.js, then exports ClassA. Base.js exports prototype methods for ClassA. Here is basically what they look like:
//ClassA/index.js
function ClassA () {
//constructor stuff
}
ClassA.prototype.constructor = ClassA;
require('./base')(ClassA);
module.exports = ClassA;
//ClassA/base.js
module.exports = ClassA => {
ClassA.prototype.aMethod = function () {
log('hello');
}
//utils.js
module.exports = {
log : function (logText) {
//dostuff
}
}
So my problem is that I cannot access the log function from within methods in ClassA/base.js. If I set const log = require('../utils').log at the top of the file, it doesn't work. It also doesn't work if I place that same line of code within the exports but outside of the method definitions, however it does work if I place it within the method itself (as expected, but this would mean replicating every require statement in any method in which it's needed). Is there any way around using this messy and repetitive route?
It was an issue with cyclic dependencies. I changed 'log' to be a method of ClassA and this resolved the issue. Thanks to Francois P. for pointing me in the right direction.
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;
From a performance perspective, is there any difference between invoking code by wrapping it in a function and then exporting it:
function doSomething () {
// doing something here
}
module.exports = doSomething();
And just requiring it without any exports? like this:
myModule.js
// Code doing something
file that requires the module:
var doSomething = require('./myModule');
And if the purpose of the code inside the module is to run just once, do I need to store it in a variable?
If you don't need the return value of that function, then you don't have to store it in a variable.
The difference with:
function doSomething () {
// doing something here
}
module.exports = doSomething();
and using:
var x = require('module');
var y = require('module');
vs.
function doSomething () {
// doing something here
}
module.exports = doSomething;
and using:
var x = require('module')();
var y = require('module')();
is that in the first case, the function will be run only once, while in the second case the function will be run twice.
The difference is that if you just include it without module.exports, then the code will execute immediately but be private to the module. You can only access the data if you export it somehow, with module.exports. It can be either a function or a Javascript Object. Essentially, you can view everything within the module as being completely hidden from everything else in your application.
The only shortcut that I know of is for JSON files. If you look here: Module.exports vs plain json for config files, you can see that you can require('file.json') and it will replace the contents of the json file with a Javascript object that you can then use in your application.
I'm developing an application with node.js and socket.io. I have built a small-scale project with some variables and functions inside a connection-specific block. These functions have access to the variables declared within that block without the need to pass the values in specifically.
This works fine, and is acceptable for a project of this size. However, as I was trying to clean the code up a bit, I looked into factoring those functions out into their own file and found modules declared by using exports.functionname as described here: http://nodejs.org/docs/v0.3.2/api/modules.html
However, these functions do not have access to the variables within the same block as they normally do when being require()'d in instead of actually being declared in the file.
Is there some way to make functions in an external file behave as if they were declared locally in nodejs?
There isn't without hacking the module system, which is exactly what I did. https://github.com/Benvie/Node.js-Ultra-REPL/blob/master/lib/ScopedModule.js
I can't say I recommend it for production. Basically the issue is more with JavaScript itself. Node wraps modules in a function so they have their own private scope. The only way to share in that scope is to be executed inside that function which wouldn't really work for a module (modular...) system. The only other scope is global which also isn't desirable.
The trick I used to get around it is in changing that wrapper function to have dynamic properties based on an externally defined set of module imports so that from inside the module wrapper all those parameters look like they're magically defined but aren't global.
Node's module wrapper looks like this:
(function (exports, module, require, __filename, __dirname){
/**module**/
})
Where mine simply adds more parameters and ensures they're resolved before executing the module wrapper.
I have it read a file named ".deps.json" in a given folder before loading a module.
An example one would be like this
[{
"providers": [ "./utility/",
"./settings/" ],
"receivers": [ "*" ]
}]
So it will load the modules in those subfolders and then expose each one as a parameter in the wrapper, named based on the filename.
The usual and clean way would be to define a function in your module, that takes the required variables as parameters:
// extracted_file.js
exports.handler = function(param1, param2) {
return param1 + param2;
}
// caller file
var extractedHandler = require('./extracted_file').handle;
var localVar1 = 1300,
localVar2 = 37;
console.log(extractedHandler(localVar1, localVar2));
You can change function scope using toString() and the dreaded eval.
If your functions were in say lib.js as follows:-
var lib = {}
lib.foo = function() { console.log(var1) }
lib.bar = function() { console.log(var2) }
module.exports = lib
In main.js you could have this:-
var var1 = 1
var var2 = 2
var ob1 = require('./lib')
ob1.foo()
ob1.bar()
When you run it the ob1.foo() line gives a ReferenceError, var1 is not defined. This is because the scope of foo() comes from lib.js and not your main js file, and var1 is invisible to it. ob1.foo is a reference to a function with lib.js scope. You need to make a new reference with main.js scope.
To do this convert each function to string and eval it. That will create a new function with the same code but with main.js scope. Here's one way using a setMainScope method which loops through all the functions changing their scope to main.js.
var var1 = 1
var var2 = 2
var ob1 = {}
ob1.setMainScope = function(ob2) {
for(var prop in ob2) {
if( !ob2.hasOwnProperty(prop) ) {continue}
if( typeof ob2[prop] !== 'function' ) {continue}
this[prop] = eval( "(" + ob2[prop].toString() + ")" )
}
}
ob1.setMainScope( require('./lib') )
ob1.foo() // 1
ob1.bar() // 2
Now it all works nicely except eval was used. The scope of foo() and bar() is now main.js and so they 'see' var1 and var2.