Node js - use of module exports as a constructor

Question

According to the Node js manual      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    The example given is      file  square js module exports   function width      return       area  function           return width   width                 and used like this   var square   require    square js    var mySquare   square 2   console log  The area of my square is     mySquare area       My question  why does the example not use square as an object  Is the following valid and does it make the example more  object oriented    var Square   require    square js    var mySquare   new Square 2   console log  The area of my square is     mySquare area

User · Answer

The example code is   in main   square width function  data       console log data squareVal         using the following may works  exports square   function width callback         var aa   new Object         callback aa squareVal   width   width

User · Answer

This question doesn t really have anything to do with how require   works   Basically  whatever you set module exports to in your module will be returned from the require   call for it   This would be equivalent to   var square   function width      return       area  function           return width   width                 There is no need for the new keyword when calling square   You aren t returning the function instance itself from square  you are returning a new object at the end   Therefore  you can simply call this function directly   For more intricate arguments around new  check this out   Is JavaScript  39 s  quot new quot  keyword considered harmful

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At the end  Node is about Javascript  JS has several way to accomplished something  is the same thing to get an  constructor   the important thing is to return a function    This way actually you are creating a new function  as we created using JS on Web Browser environment for example     Personally i prefer the prototype approach  as Sukima suggested on this post  Node js - use of module exports as a constructor

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CommonJS modules allow two ways to define exported properties  In either case you are returning an Object Function  Because functions are first class citizens in JavaScript they to can act just like Objects  technically they are Objects   That said your question about using the new keywords has a simple answer  Yes  I ll illustrate     Module exports  You can either use the exports variable provided to attach properties to it  Once required in another module those assign properties become available  Or you can assign an object to the module exports property  In either case what is returned by require   is a reference to the value of module exports   A pseudo-code example of how a module is defined   var theModule       exports          function module  exports  require          Your module code goes here     theModule  theModule exports  theRequireFunction     In the example above module exports and exports are the same object  The cool part is that you don t see any of that in your CommonJS modules as the whole system takes care of that for you all you need to know is there is a module object with an exports property and an exports variable that points to the same thing the module exports does   Require with constructors  Since you can attach a function directly to module exports you can essentially return a function and like any function it could be managed as a constructor  That s in italics since the only difference between a function and a constructor in JavaScript is how you intend to use it  Technically there is no difference    So the following is perfectly good code and I personally encourage it      My module function MyObject bar      this bar   bar     MyObject prototype foo   function foo       console log this bar       module exports   MyObject      In another module  var MyObjectOrSomeCleverName   require    my object js    var my obj instance   new MyObjectOrSomeCleverName  foobar    my obj instance foo         gt   foobar    Require for non-constructors  Same thing goes for non-constructor like functions      My Module exports someFunction   function someFunction msg      console log msg         In another module var MyModule   require    my module js    MyModule someFunction  foobar         gt   foobar

User · Answer

In my opinion  some of the node js examples are quite contrived   You might expect to see something more like this in the real world     square js function Square width       if    this instanceof Square         return new Square width          this width   width      Square prototype area   function area       return Math pow this width  2       module exports   Square    Usage  var Square   require    square        you can use  new  keyword var s   new Square 5   s area       25     or you can skip it  var s2   Square 10   s2 area       100     For the ES6 people  class Square     constructor width        this width   width        area         return Math pow this width  2          export default Square    Using it in ES6  import Square from    square            When using a class  you must use the new keyword to instatiate it  Everything else stays the same

[javascript] Node.js - use of module.exports as a constructor

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