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;
What is the difference between Node's module.exports and ES6's export default? I'm trying to figure out why I get the "__ is not a constructor" error when I try to export default in Node.js 6.2.2.
What works
'use strict'
class SlimShady {
constructor(options) {
this._options = options
}
sayName() {
return 'My name is Slim Shady.'
}
}
// This works
module.exports = SlimShady
What doesn't work
'use strict'
class SlimShady {
constructor(options) {
this._options = options
}
sayName() {
return 'My name is Slim Shady.'
}
}
// This will cause the "SlimShady is not a constructor" error
// if in another file I try `let marshall = new SlimShady()`
export default SlimShady
The issue is with
how ES6 modules are emulated in CommonJS
how you import the module
ES6 to CommonJS
At the time of writing this, no environment supports ES6 modules natively. When using them in Node.js you need to use something like Babel to convert the modules to CommonJS. But how exactly does that happen?
Many people consider module.exports = ... to be equivalent to export default ... and exports.foo ... to be equivalent to export const foo = .... That's not quite true though, or at least not how Babel does it.
ES6 default exports are actually also named exports, except that default is a "reserved" name and there is special syntax support for it. Lets have a look how Babel compiles named and default exports:
// input
export const foo = 42;
export default 21;
// output
"use strict";
Object.defineProperty(exports, "__esModule", {
value: true
});
var foo = exports.foo = 42;
exports.default = 21;
Here we can see that the default export becomes a property on the exports object, just like foo.
Import the module
We can import the module in two ways: Either using CommonJS or using ES6 import syntax.
Your issue: I believe you are doing something like:
var bar = require('./input');
new bar();
expecting that bar is assigned the value of the default export. But as we can see in the example above, the default export is assigned to the default property!
So in order to access the default export we actually have to do
var bar = require('./input').default;
If we use ES6 module syntax, namely
import bar from './input';
console.log(bar);
Babel will transform it to
'use strict';
var _input = require('./input');
var _input2 = _interopRequireDefault(_input);
function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
console.log(_input2.default);
You can see that every access to bar is converted to access .default.
Felix Kling did a great comparison on those two, for anyone wondering how to do an export default alongside named exports with module.exports in nodejs
module.exports = new DAO()
module.exports.initDAO = initDAO // append other functions as named export
// now you have
let DAO = require('_/helpers/DAO');
// DAO by default is exported class or function
DAO.initDAO()
You need to configure babel correctly in your project to use export default and export const foo
npm install --save-dev #babel/plugin-proposal-export-default-from
then add below configration in .babelrc
"plugins": [
"#babel/plugin-proposal-export-default-from"
]
If I have this ts module:
export function say(){
console.log("said");
}
and I compile it with the amd option I can use it quite easily from a ts client :
import foo = module("tsmodule")
foo.say();
export var x = 123;
However if I have javascript equivalent to the ts module:
define(["require", "exports"], function(require, exports) {
function say() {
console.log("said");
}
exports.say = say;
})
There is no way to use it easily. The simplest possible solution:
// of course you can use .d.ts for requirejs but that is beside the point
declare var require:any;
// will fail with error module has not been loaded yet for context
// http://requirejs.org/docs/errors.html#notloaded
var useme = require("jsmodule")
useme.say();
export var x = 123;
import foo = module("tsmodule")
foo.say();
fails because of error http://requirejs.org/docs/errors.html#notloaded . Since "jsmodule" was not passed to the define call in the generated typescript.
The two workarounds I have
don't use import / export (language features lost)
use require([]) (still can't export something that depends on the require([]) call)
have limitations : https://github.com/basarat/typescript-requirejs . Is there another way? If not can you vote here : https://typescript.codeplex.com/workitem/948 :)
If you want to load in a JavaScript module you could always use the (badly documented) amd-dependency tag:
/// <amd-dependency path="jsmodule" />
This will put jsmodule in the dependency array of your define call.
And then provide a declaration file in which you would simply state
module useme {
function say(): void;
}
I'm trying to use typescript with angularjs in the client-side.
I found if I use external modules, the generated js won't be run in browser.
controllers.ts
/// <reference path="./libs/underscore.d.ts"/>
import _ = module("underscore");
module test {
export class Ctrl {
constructor($scope:any) {
$scope.name = "Freewind";
_.each($scope.name, function(item) {});
}
}
}
The generated js will be:
var _ = require("underscore")
var test;
(function (test) {
var Ctrl = (function () {
function Ctrl($scope) {
$scope.name = "Freewind";
_.each($scope.name, function (item) {
});
}
return Ctrl;
})();
test.Ctrl = Ctrl;
})(test || (test = {}));
Which can't run correctly. But if I remove the module("underscore") part, it will be OK.
Since I have add underscore.js in the HTML, I think it should be something wrong with the require() method. How to fix it?
There are two ways to load stuff in your HTML pages.
Bundling
The first one is to manually include all the script files in your page. You might run some kind of pre-release step to merge and minify your code - but you're taking responsibility for that instead of leaving it to the code to do. This is generally called bundling.
In the case on bundling, you only use references in your TypeScript code (not imports), like this:
/// <reference path="./libs/underscore.d.ts"/>
module test {
export class Ctrl {
constructor($scope:any) {
$scope.name = "Freewind";
_.each($scope.name, function(item) {});
}
}
}
Module loading
If you want to use a module loader, which for The Web is typically RequireJS, you can load External Modules using the import statement. Normally you wouldn't need the reference in this case...
import _ = module("./libs/underscore");
module test {
export class Ctrl {
constructor($scope:any) {
$scope.name = "Freewind";
_.each($scope.name, function(item) {});
}
}
}
RequireJS with non-modules
There is a third scenario, which is quite common. If you intend to import something that isn't an External Module (such as jQuery, but underscore may also fit this pattern), you are better off using a reference and a manual call to RequireJS.
RequireJS will load the dependency for you, so you would wrap your main program with it (which would probably be in a separate file such as app.ts.
///<reference path="./libs/require.d.ts" />
///<reference path="./libs/underscore.d.ts" />
module test {
export class Ctrl {
constructor($scope:any) {
$scope.name = "Freewind";
_.each($scope.name, function(item) {});
}
}
}
require(['underscore'], function (_) {
var ctrl = new test.Crtl({});
});
You can also use require.config to specify the path to underscore in your application.
require.config({
paths: {
"underscore": "libs/underscore"
}
});
When you use require, you asserting that underscore is exported as a loadable module. This further assumes that you are using some sort of module loader system (TypeScript currently supports AMD and CommonJS modules). Since you aren't using a module system, and underscore is simply available in global scope, you can use a /// reference to tell TypeScript that Underscore is available in global scope. Put the following at the top of your file:
/// <reference path="./libs/underscore.d.ts">
And you should be good to go!
In AMD (as implemented in requirejs) one can defined modules to be included as dependencies, eg:
define(['require','exports'], function(require, exports) {
var externalDep = require('path/to/depModule');
// Use the module somewhere.
});
I have tried the --module amd and it outputs correctly an AMD module usable by requirejs.
Is it possible to define dependencies inside the source of TypeScript source file that translates to something like the example above?
You need to "export" your modules;
export module depModule {
export class A {
}
}
that will transalate into JavaScript code that looks like:
define(["require", "exports"], function(require, exports) {
(function (depModule) {
var A = (function () {
function A() { }
return A;
})();
depModule.A = A;
})(exports.depModule || (exports.depModule = {}));
})
and then you consume them by using "import":
module otherModule {
import depModule = module('depModule');
var a = new depModule.depModule.A();
}
you will need to specify the type of your module code generation to the compiler using --module AMD.