module exports vs exports in Node js

Question

I ve found the following contract in a Node js module   module exports   exports   nano   function database module cfg          I wonder whats the different between module exports and exports and why both are used here

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Each file you create is a module  module is an object  It has property called exports      which is empty object by default   you can create functions middlewares and add  to this empty exports object such as exports findById     gt            then require anywhere in your app and use     controllers user js  exports findById        gt            do something     require in routes js to use   const  findyId       controllers user

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I went through some tests and I think this may shed some light on the subject     app js   var         routes   require    routes              console log   routes   routes         versions of  routes index js   exports   function fn         outputs   routes      exports fn   function fn          outputs   routes   fn   Function  fn      module exports   function fn          outputs   routes function fn       module exports fn   function fn          outputs   routes   fn   Function  fn       I even added new files     routes index js   module exports   require    not-index js    module exports   require    user js        routes not-index js   exports   function fn          routes user js   exports   function user        We get the output   routes          routes index js   module exports fn   require    not-index js    module exports user   require    user js        routes not-index js   exports   function fn          routes user js   exports   function user        We get the output   routes   fn      user             routes index js   module exports fn   require    not-index js    module exports user   require    user js        routes not-index js   exports fn   function fn          routes user js   exports user   function user        We get the output   routes   user   Function  user     If we change user js to   ThisLoadedLast   Function  ThisLoadedLast     we get the output   routes   ThisLoadedLast   Function  ThisLoadedLast         But if we modify   routes index js       routes index js   module exports fn   require    not-index js    module exports ThisLoadedLast   require    user js        routes not-index js   exports fn   function fn          routes user js   exports ThisLoadedLast   function ThisLoadedLast            we get   routes   fn    fn   Function  fn     ThisLoadedLast    ThisLoadedLast   Function  ThisLoadedLast        So I would suggest always use module exports in your module definitions   I don t completely understand what s going on internally with Node  but please comment if you can make more sense of this as I m sure it helps   -- Happy coding

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From the docs  The exports variable is available within a module s file-level scope  and is assigned the value of module exports before the module is evaluated  It allows a shortcut  so that module exports f       can be written more succinctly as exports f        However  be aware that like any variable  if a new value is assigned to exports  it is no longer bound to module exports   It is just a variable pointing to module exports

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If you want the root of your module s export to be a function  such as a constructor  or if you want to export a complete object in one assignment instead of building it one property at a time  assign it to module exports instead of exports   - http   nodejs org api modules html

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Here is the result of   console log  module     console log module    console log  exports     console log exports    console log  module exports     console log module exports       Also   if module exports     exports       console log  YES     else      console log  NO         YES   Note  The CommonJS specification only allows the use of the exports variable to expose public members  Therefore  the named exports pattern is the only one that is really compatible with the CommonJS specification  The use of module exports is an extension provided by Node js to support a broader range of module definition patterns

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var a      md         Firstly the exports and module exports point the same empty Object  exp   a   exports  a  md exp   a   module exports   a   exp attr    change    console log md exp     attr  change       If you point exp to other object instead of point it s property to other object  The md exp will be empty Object     var a     md     exp  a  md exp  a   exp   function    console log  Do nothing           console log md exp

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JavaScript passes objects by copy of a reference  It s a subtle difference to do with the way objects are passed by reference in JavaScript   exports and module exports both point to the same object  exports is a variable and module exports is an attribute of the module object   Say I write something like this   exports    a 1   module exports    b 12     exports and module exports now point to different objects  Modifying exports no longer modifies module exports   When the import function inspects module exports it gets  b 12

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Basically the answer lies in what really happens when a module is required via require statement  Assuming this is the first time the module is being required   For example   var x   require  file1 js      contents of file1 js   module exports    123     When the above statement is executed  a Module object is created  Its constructor function is   function Module id  parent        this id   id      this exports           this parent   parent      if  parent  amp  amp  parent children            parent children push this              this filename   null      this loaded   false      this children           As you see each module object has a property with name exports  This is what is eventually returned as part of require   Next step of require is to wrap the contents of file1 js into an anonymous function like below     function  exports  require  module    filename    dirname           contents from file1 js     module exports    123        And this anonymous function is invoked the following way  module here refers to the Module Object created earlier    function  exports  require  module    filename    dirname           contents from file1 js     module exports    123      module exports require  module   path to file1 js   directory of the file1 js      As we can see inside the function  exports formal argument refers to module exports  In essence it s a convenience provided to the module programmer   However this convenience need to be exercised with care  In any case if trying to assign a new object to exports ensure we do it this way   exports   module exports         If we do it following way wrong way  module exports will still be pointing to the object created as part of module instance   exports         As as result adding anything to the above exports object will have no effect to module exports object and nothing will be exported or returned as part of require

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in node js module js file is use to run the module load system every time when node execute a file it wrap your js file content as follow    function  exports  require  module    filename    dirname              your js file content    n       because of this wrapping inside ur js source code you can access exports require module etc   this approach is used because there is no other way to get functionalities wrote in on js file to another   then node execute this wrapped function using c    at that moment exports object that passed into this function will be filled   you can see inside this function parameters exports and module  actually exports is a public member of module constructor function   look at following code  copy this code into b js  console log  module is   Object prototype toString call module    console log  object keys   Object keys module    console log module exports   console log exports     module exports   console log  exports is   Object prototype toString call exports    console log  ----------------------------------------------    var foo   require  a js    console log  object keys of foo    Object keys foo    console log  name is    foo   foo      copy this code to a js  exports name    hello   module exports name    hi   module exports age   23  module exports   function   console log  function to module exports       exports   function   console log  function to export       now run using node  this is the output  module is  object Object  object keys id exports parent filename loaded children paths    true       exports is  object Object   object keys of foo   name is function    console log  function to module exports    function to module exports  now remove the commented line in a js and comment the line above that line and remove the last line of b js and run   in javascript world you cannot reassign object that passed as parameter but you can change function s public member when object of that function set as a parameter to another function  do remember  use module exports on and only if you wants to get a function when you use require keyword   in above example we var foo   require a js   you can see we can call foo as a function   this is how node documentation explain it  The exports object is created by the Module system  Sometimes this is not acceptable  many want their module to be an instance of some class  To do this assign the desired export object to module exports

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This shows how require   works in its simplest form  excerpted from Eloquent JavaScript  Problem It is not possible for a module to directly export a value other than the exports object  such as a function  For example  a module might want to export only the constructor of the object type it defines  Right now  it cannot do that because require always uses the exports object it creates as the exported value   Solution Provide modules with another variable  module  which is an object that has a property exports  This property initially points at the empty object created by require but can be overwritten with another value in order to export something else   function require name      if  name in require cache      return require cache name     var code   new Function  exports  module   readFile name      var exports       module    exports  exports     code exports  module     require cache name    module exports    return module exports    require cache   Object create null

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Initially module exports exports   and the require function returns the object module exports refers to   if we add property to the object  say exports a 1  then module exports and exports still refer to the same object  So if we call require and assign the module to a variable  then the variable has a property a and its value is 1   But if we override one of them  for example  exports function      then they are different now  exports refers to a new object and module exports refer to the original object  And if we require the file  it will not return the new object  since module exports is not refer to the new object   For me  i will keep adding new property  or override both of them to a new object  Just override one is not right  And keep in mind that module exports is the real boss

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Both module exports and exports point to the same function database module cfg           1  var a  b  2  a   b   function     console log  Old       3      b   function     console log  New       4  5  a        Old  6  b        New    You can change b on line 3 to a  the output is reverse  The conclusion is      a and b are independent   So module exports   exports   nano   function database module cfg        is equivalent to   var f   function database module cfg         module exports   f  exports   f    Assumed the above is module js  which is required by foo js  The benefits of module exports   exports   nano   function database module cfg        is clear now    In foo js  since module exports is require    module js     var output   require    modules js       In moduls js  You can use exports instead of module exports     So  you will be happy if both exports and module exports pointing to the same thing

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I found this link useful to answer the above question   http   timnew me blog 2012 04 20 exports-vs-module-exports-in-node-js   To add to the other posts  The module system in node does      var exports   module exports    before executing your code  So when you want to exports   foo    you probably want to do module exports   exports   foo  but using exports foo   foo should be fine

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I just make some test  it turns out that  inside nodejs s module code  it should something like this   var module exports       var exports   module exports    so   1   exports   function         this will not work  as it make the exports to some other pointer module exports   function         it works  cause finally nodejs make the module exports to export    2   exports abc   function         works  exports efg   function         works    3  but  while in this case  module exports   function         from now on we have to using module exports to attach more stuff to exports  module exports a    value a      works exports b    value b      the b will nerver be seen cause of the first line of code we have do it before  or later

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exports and module exports are the same unless you reassign exports within your module   The easiest way to think about it  is to think that this line is implicitly at the top of every module   var exports   module exports         If  within your module  you reassign exports  then you reassign it within your module and it no longer equals module exports  This is why  if you want to export a function  you must do    module exports   function             If you simply assigned your function           to exports  you would be reassigning exports to no longer point to module exports   If you don t want to refer to your function by module exports every time  you can do   module exports   exports   function             Notice that module exports is the left most argument   Attaching properties to exports is not the same since you are not reassigning it  That is why this works  exports foo   function

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Setting module exports allows the database module function to be called like a function when required   Simply setting exports wouldn t allow the function to be exported because node exports the object module exports references   The following code wouldn t allow the user to call the function  module js The following won t work  exports   nano   function database module cfg   return    The following will work if module exports is set  module exports   exports   nano   function database module cfg   return    console var func   require    module js       the following line will   work   with module exports func     Basically node js doesn t export the object that exports currently references  but exports the properties of what exports originally references   Although Node js does export the object module exports references  allowing you to call it like a function   2nd least important reason They set both module exports and exports to ensure exports isn t referencing the prior exported object   By setting both you use exports as a shorthand and avoid potential bugs later on down the road  Using exports prop   true  instead of module exports prop   true saves characters and avoids confusion

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To understand the differences  you have to first understand what Node js does to every module during runtime  Node js creates a wrapper function for every module     function exports  require  module    filename    dirname             Notice the first param exports is an empty object  and the third param module is an object with many properties  and one of the properties is named exports  This is what exports comes from and what module exports comes from  The former one is a variable object  and the latter one is a property of module object   Within the module  Node js automatically does this thing at the beginning  module exports   exports  and ultimately returns module exports   So you can see that if you reassign some value to exports  it won t have any effect to module exports   Simply because exports points to another new object  but module exports still holds the old exports   let exports       const module       module exports   exports   exports     a  1   console log module exports          But if you updates properties of exports  it will surely have effect on module exports  Because they both point to the same object   let exports       const module       module exports   exports   exports a   1  module exports b   2  console log module exports       a  1  b  2     Also notice that if you reassign another value to module exports  then it seems meaningless for exports updates  Every updates on exports is ignored because module exports points to another object   let exports       const module       module exports   exports   exports a   1  module exports       hello       gt  console log  hello     console log module exports       hello       gt  console log  hello

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Here is a good description written about node modules in node js in action book from Manning publication  What ultimately gets exported in your application is module exports  exports is set up simply as a global reference to module exports   which initially is defined as an empty object that you can add properties to  So exports myFunc is just shorthand for module exports myFunc  As a result  if exports is set to anything else  it breaks the reference between module exports and exports   Because module exports is what really gets exported  exports will no longer work as expected   it doesn   t reference module  exports anymore  If you want to maintain that link  you can make module exports reference exports  again as follows   module exports   exports   db

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exports  it s a reference to module exports object both exports and module exports point to the same object until we change the reference of exports object  Example   if exports a   10 then module exports a   10  if we reassign exports object explicitly inside the code like exports      now its lost the reference to module exports

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1 exports -  use as singleton utility  2  module-exports -  use as logical objects such as service   model etc

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why both are used here  I believe they just want to be clear that module exports  exports  and nano point to the same function - allowing you to use either variable to call the function within the file  nano provides some context to what the function does  exports won t be exported  only module exports will   so why bother overwriting that as well  The verbosity trade-off limits the risk of future bugs  such as using exports instead of module exports within the file  It also provides clarification that module exports and exports are in fact pointing to the same value   module exports vs exports As long as you don t reassign module exports or exports  and instead add values to the object they both refer to   you won t have any issues and can safely use exports to be more concise  When assigning either to a non-object  they are now pointing to different places which can be confusing unless you intentionally want module exports to be something specific  such as a function   Setting exports to a non-object doesn t make much sense as you ll have to set module exports   exports at the end to be able to use it in other files  let module     exports        let exports   module exports   exports msg    hi   console log module exports     exports      true  exports    yo   console log module exports     exports      false  exports   module exports  console log module exports     exports      true  module exports    hello   console log module exports     exports      false  module exports   exports  console log module exports     exports      true  Why assign module exports to a function  More concise  Compare how much shorter the 2nd example is  helloWorld1 js  module exports hello        gt  console log  hello world    app1 js  let sayHello   require    helloWorld1    sayHello hello     hello world helloWorld2 js  module exports        gt  console log  hello world    app2 js  let sayHello   require    helloWorld2    sayHello     hello world

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Even though question has been answered and accepted long ago  i just want to share my 2 cents   You can imagine that at the very beginning of your file there is something like  just for explanation    var module   new Module       var exports   module exports      So whatever you do just keep in mind that module exports and NOT exports will be returned from your module when you re requiring that module from somewhere else   So when you do something like   exports a   function         console log  a      exports b   function         console log  b        You are adding 2 function a and b to the object on which module exports points too  so the typeof the returning result will be an object     a   Function   b   Function     Of course  this is the same result you will get if you are using module exports in this example instead of exports   This is the case where you want your module exports to behave like a container of exported values  Whereas  if you only want to export a constructor function then there is something you should know about using module exports or exports  Remember again that module exports will be returned when you require something  not export    module exports   function Something         console log  bla bla        Now typeof returning result is  function  and you can require it and immediately invoke like   var x   require    file1 js      because you overwrite the returning result to be a function   However  using exports you can t use something like   exports   function Something         console log  bla bla      var x   require    file1 js        Error  require is not a function   Because with exports  the reference doesn t point anymore to the object where module exports points  so there is not a relationship between exports and module exports anymore  In this case module exports still points to the empty object    which will be returned   Accepted answer from another topic should also help  Does Javascript pass by reference

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module exports and exports both point to the same object before the module is evaluated    Any property you add to the module exports  object will be available when your module is used in another module using require statement  exports is a shortcut made available for the same thing  For instance    module exports add    a  b    gt  a b   is equivalent to writing   exports add    a  b    gt  a b   So it is okay as long as you do not assign a new value to exports variable  When you do something like this   exports    a  b    gt  a b    as you are assigning a new value to exports it no longer has reference to the exported object and thus will remain local to your module   If you are planning to assign a new value to module exports rather than adding new properties to the initial object made available  you should probably consider doing as given below   module exports   exports    a  b    gt  a b   Node js website has a very good explanation of this

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Let s create one module with 2 ways   One way  var aa         a       gt   return  a        b       gt   return  b      module exports   aa    Second way  exports a        gt   return  a    exports b        gt   return  b      And this is how require   will integrate module   First way   function require        module exports           var exports   module exports       var aa             a       gt   return  a            b       gt   return  b             module exports   aa       return module exports      Second way  function require        module exports           var exports   module exports       exports a        gt   return  a        exports b        gt   return  b         return module exports

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