What is the purpose of Node js module exports and how do you use it

Question

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

User · Answer

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");

User · Answer

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

User · Answer

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        gt   console log  func1      exports Func2        gt   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

User · Answer

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  amp  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

User · Answer

let test   function         return  Hello world     exports test   test

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

[javascript] What is the purpose of Node.js module.exports and how do you use it?

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