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TypeScript organization: namespaces? modules? confusion

I'm new to TypeScript and the more I read about modules and namespaces, the more it confuses me. Should I go with modules? Should I go with namespaces? should I use both? help!
I have existing javascript (.js) files that I'm trying to convert to TypeScript.
There is one .js file with some general functions and one .js file with some functions specific to filters.
Now I would like to organize this a bit more with TypeScript, as I would normally do with C#.
Is this correct usage or how should it be organized?
I'm not using a module, should I? (how?)
Company.ts
namespace Company {
// nothing yet, but in future it might.
}
Company.Project.ts
namespace Company.Project {
import Company; // like this?
let myVar : string = "something";
export function handyGeneralFunction1(foo, bar) {
// ...
}
export function handyGeneralFunction2(foo, bar, foo, bar) {
// ...
doInternalCalc();
// ...
}
export function handyGeneralFunction3() {
// ...
}
function doInternalCalc() {
// ...
}
}
Company.Project.Filter.ts
namespace Company.Project.Filter {
import Project = Company.Project; // like this?
export function initializeFilter() {
// ...
initMetadata();
// ...
}
function initMetadata() {
// ...
Project.handyGeneralFunction3();
let helper : FilterHelper = new FilterHelper("aaaa,bbbb");
let res:string[] = helper.parseData();
}
function foo() {
// ...
let x :string = Project.myVar + " else"; // can I use myVar here?
}
// a class in the namespace
export class FilterHelper {
data: string;
constructor(theData: string) {
this.data = theData;
}
parseData() : string[] {
// ...
return new Array<string>();
}
}
}

If you have the possibility to really improve the project structure, you should definitely go with modules instead of namespaces. Depending on the size of the project, this can require some effort.
Introducing namespaces could be useful if your app is not that large and you don't want to invest in switching to a different build system that can handle real modules. Using namespaces is not much more than some named global variables that contain the functions and variables. This can get confusing pretty soon as you don't have any proper dependency structures. You need to take care of importing all files in the correct order for namespaces to work correctly. The import syntax you used can even be omitted, as you anyway only reference another global object, that needs to be initialized at that point in time.
So namespaces could be your very first step if you don't have the possibility to do any larger changes on your codebase. For a future-proof setup, I would definitely suggest to use real modules. But be aware that this doesn't come for free.
For further information, I suggest to have a look at the official comment on this at https://www.typescriptlang.org/docs/handbook/namespaces-and-modules.html or the section in TypeScript Deep Dive at https://basarat.gitbooks.io/typescript/content/docs/project/namespaces.html

Testing TypeScript function which is not exported

I use Mocha/Chai for testing JavaScript front-end code and now we switched to TypeScript. I have several functions I want to test. But they shouldn't be exportable. Can I get an access to this functions and test it without adding export to them?

There is no way to access a non-exported module function.
module MyModule {
function privateFunction() {
alert("privateFunction");
}
}
MyModule.privateFunction(); // Generates a compiler error
However, leaving aside the question of validity of private method testing, here is what you can do.
Group your functions into an utility class, and then leverage the fact that private class members can be accessed via square bracket notation.
module MyModule {
export class UtilityClass {
private privateFunction() {
alert("privateFunction");
}
}
}
var utility = new MyModule.UtilityClass();
//utility.privateFunction(); Generates a compiler error
utility["privateFunction"](); // Alerts "privateFunction"

While it is not possible to access non-exported function directly there is still a way to export them in a "semi-hidden" way. One possible approach would be:
// In your library module declare internal functions as non-exported like normal.
function someInternalFunctionA(x: number): number {
return x;
}
function someInternalFunctionB(x: number): number {
return x;
}
// At the bottom, offer a public escape hatch for accessing certain functions
// you would like to be available for testing.
export const _private = {
someInternalFunctionA,
someInternalFunctionB,
};
On test side you can do:
import { _private } from "./myModule";
test("someInternalFunctionA", () => {
expect(_private.someInternalFunctionA(42)).toEqual(42);
});
What I like about the approach:
No need to mark someInternalFunctionA with export directly.
It should still be very obvious that the stuff under _private isn't officially part of the public interface.

As you can see in related questions the issue of testing private functions inside classes or modules is heavily debated on StackOverflow - The following might be an architectural solution to not even have that discussion:
If these functions feel important enough to be tested separately, but should not be accessible as part of a module, should they maybe be placed in their own module?
This would solve your issue of accessibility - they now are public functions in one module, and you can easily use them from within another module and not expose them as part of that module.

Best solution I found is to export the private function under different name so this name will remind you not to use this function anywhere else.
export const getPrice__test = getPrice;
function getPrice(): number {
return 10 + Math.Random() * 50;
}

But they shouldn't be exportable. Can I get an access to this functions and test it without adding export to them?
In general, no. It's possible to access private class members, but not unexported members of modules.
I would echo #Katana314's comments -- unit tests should not be concerning themselves with non-public methods. Trying to do so is an indication that you're testing the implementation details of the module rather than the contract the module claims to implement.

This is a total hack, but hey...
window.testing = {};
Then in your module:
module.exports = {
myPublicFunction: myPublicFunction
};
window.testing.myModule = {
myPublicFunction: myPublicFunction,
myPrivateFunction: myPrivateFunction
};

browserify circular dependency: something is not a function

I've recently started writing CommonJS modules but facing issues in requiring modules. Why is storage.js unable to reach the example module which I have required? What is the proper way to require a dependent module in this case?
EDIT: Included more information because the previous question omitted hello.js, which I thought wasn't the cause of the problem. Seems like including it causes a uncaught type error. Also, this is part of the code for a chrome extension, and main.js is the content script.
// main.js
var hello = require('./hello');
var storage = require('./storage');
var example = require('./example');
storage.store();
// storage.js
var example = require('./example');
module.exports = (function() {
function store() {example.ex();}
return {store: store};
})();
// example.js
var storage = require('./storage');
module.exports = (function() {
function ex() {
console.log('example');
}
return {ex: ex};
})();
// hello.js
var example = require('./example'); // <<<< Including this gives Uncaught TypeError: example.ex is not a function
module.exports = (function() {
function hello() {
console.log('hello');
}
return {hello:hello};
})();

Not a direct answer to your question, but Browserify will wrap in a self-invoking function for you. You can simplify your lib files:
// main.js
var storage = require('./storage');
storage.store();
Because you don't use hello or example, don't require them.
// storage.js
var example = require('./example');
function store() {example.ex();}
module.exports.store = store;
No need to go through the self-invoking function here.
// example.js
module.exports.ex = ex;
function ex() {
console.log('Example');
}
This doesn't use storage, so don't include it.
hello.js does nothing but trigger the circular dependency, remove it.
In your updated code, you have a circular dependency between storage.js and example.js. Because you don't use anything from storage in example, you can just remove that require. I still think you should remove the self-invoking functions, as that's already part of commonjs.
When loading a module, Commonjs will only execute the file a single time. Everything on module.exports is then cached for future calls. When you include the circular dependency for the first time, the module loader sees that its currently being loaded, and you don't get any results back. Subsequent calls will complete as normal.

What does exports mean in javascript? [duplicate]

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 do "module.exports" and "exports.methods" mean in NodeJS / Express?

Looking at a random source file of the express framework for NodeJS, there are two lines of the code that I do not understand (these lines of code are typical of almost all NodeJS files).
/**
* Expose `Router` constructor.
*/
exports = module.exports = Router;
and
/**
* Expose HTTP methods.
*/
var methods = exports.methods = require('./methods');
I understand that the first piece of code allows the rest of the functions in the file to be exposed to the NodeJS app, but I don't understand exactly how it works, or what the code in the line means.
What do exports and module.exports actually mean?
I believe the 2nd piece of code allows the functions in the file to access methods, but again, how exactly does it do this.
Basically, what are these magic words: module and exports?

To be more specific:
module is the global scope variable inside a file.
So if you call require("foo") then :
// foo.js
console.log(this === module); // true
It acts in the same way that window acts in the browser.
There is also another global object called global which you can write and read from in any file you want, but that involves mutating global scope and this is EVIL
exports is a variable that lives on module.exports. It's basically what you export when a file is required.
// foo.js
module.exports = 42;
// main.js
console.log(require("foo") === 42); // true
There is a minor problem with exports on it's own. The _global scope context+ and module are not the same. (In the browser the global scope context and window are the same).
// foo.js
var exports = {}; // creates a new local variable called exports, and conflicts with
// living on module.exports
exports = {}; // does the same as above
module.exports = {}; // just works because its the "correct" exports
// bar.js
exports.foo = 42; // this does not create a new exports variable so it just works
Read more about exports

To expand on Raynos's answer...
exports is basically an alias for module.exports - I recommend just not using it. You can expose methods and properties from a module by setting them on module.exports, as follows:
//file 'module1.js'
module.exports.foo = function () { return 'bar' }
module.exports.baz = 5
Then you get access to it in your code:
var module1 = require('module1')
console.log(module1.foo())
console.log(module1.baz)
You can also override module.exports entirely to simply provide a single object upon require:
//glorp.js
module.exports = function () {
this.foo = function () { return 'bar' }
this.baz = 5
return this // need to return `this` object here
}
Now you've got a nice prototype:
var g1 = new require('glorp')()
console.log(g1.foo())
console.log(g1.baz)
There are myriad other ways to play with module.exports and require. Just remember, require('foo') always returns the same instance even if you call it multiple times.
Note
For the following to work,
var g1 = new require('glorp')()
console.log(g1.foo())
console.log(g1.baz)
this has to be returned in the function that is assigned to module.exports. Otherwise, you'll get a TypeError:
console.log(g1.foo())
^
TypeError: Cannot read property 'foo' of undefined

You can find the best answer in node.js source code.
If someone is requiring your js module,
your script turns into a function by node as follows (see src/node.js).
// require function does this..
(function (exports, require, module, __filename, __dirname) {
... your javascript contents...
});
Node will wrap your script. Then above script will be executed as follows:
//module.js
var args = [self.exports, require, self, filename, dirname];
return compiledWrapper.apply(self.exports, args);
So in your script,
exports is just module.exports.
In your script, you can add something to this exports object (functions..).
require function will return this object. This is node.js's module system (commonJS specification).
But be careful not to modify module.exports. Otherwise your current exports will be meaningless.

module is an object that represents what that particular source file would like to publicly expose. Instead of having something akin to header files in the c/c++ world, you describe what the module exports by defining this object. the node runtime then uses this object to determine what about your module is 'public.'
its a similar concept to exporting functions from a dll in the compiled world. you have to define explicitly what functions can be accessed by the outside world. this helps with encapsulation and lets your organize your libraries in a clean way.

Module's code is wrapped in module.exports (The module, maybe composed by other module).
There are many ways to build a module, but this is one very common (and my personal favorite).
// Dependencies
// const module = require('module');
// Module object
var foo = {}
// Internal property
foo._a = 'a';
// "Public" property
foo.b = 'b';
// Method
foo.fu = function() { return 'fu' };
// Export
module.exports = foo;