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 what

Answer 1

1. 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.

2. 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.

Answer 2

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}

Answer 3

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

Answer 4

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() { ... }

Answer 5

"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

Answer 6

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);