Develop Reference

why exports returns undefined however module.exports returns the right data [duplicate]

On this page (http://docs.nodejitsu.com/articles/getting-started/what-is-require), it states that "If you want to set the exports object to a function or a new object, you have to use the module.exports object."
My question is why.
// right
module.exports = function () {
console.log("hello world")
}
// wrong
exports = function () {
console.log("hello world")
}
I console.logged the result (result=require(example.js)) and the first one is [Function] the second one is {}.
Could you please explain the reason behind it? I read the post here: module.exports vs exports in Node.js . It is helpful, but does not explain the reason why it is designed in that way. Will there be a problem if the reference of exports be returned directly?

module is a plain JavaScript object with an exports property. exports is a plain JavaScript variable that happens to be set to module.exports.
At the end of your file, node.js will basically 'return' module.exports to the require function. A simplified way to view a JS file in Node could be this:
var module = { exports: {} };
var exports = module.exports;
// your code
return module.exports;
If you set a property on exports, like exports.a = 9;, that will set module.exports.a as well because objects are passed around as references in JavaScript, which means that if you set multiple variables to the same object, they are all the same object; so then exports and module.exports are the same object.
But if you set exports to something new, it will no longer be set to module.exports, so exports and module.exports are no longer the same object.

Renee's answer is well explained. Addition to the answer with an example:
Node does a lot of things to your file and one of the important is WRAPPING your file. Inside nodejs source code "module.exports" is returned. Lets take a step back and understand the wrapper. Suppose you have
greet.js
var greet = function () {
console.log('Hello World');
};
module.exports = greet;
the above code is wrapped as IIFE(Immediately Invoked Function Expression) inside nodejs source code as follows:
(function (exports, require, module, __filename, __dirname) { //add by node
var greet = function () {
console.log('Hello World');
};
module.exports = greet;
}).apply(); //add by node
return module.exports; //add by node
and the above function is invoked (.apply()) and returned module.exports.
At this time module.exports and exports pointing to the same reference.
Now, imagine you re-write
greet.js as
exports = function () {
console.log('Hello World');
};
console.log(exports);
console.log(module.exports);
the output will be
[Function]
{}
the reason is : module.exports is an empty object. We did not set anything to module.exports rather we set exports = function()..... in new greet.js. So, module.exports is empty.
Technically exports and module.exports should point to same reference(thats correct!!). But we use "=" when assigning function().... to exports, which creates another object in the memory. So, module.exports and exports produce different results. When it comes to exports we can't override it.
Now, imagine you re-write (this is called Mutation)
greet.js (referring to Renee answer) as
exports.a = function() {
console.log("Hello");
}
console.log(exports);
console.log(module.exports);
the output will be
{ a: [Function] }
{ a: [Function] }
As you can see module.exports and exports are pointing to same reference which is a function. If you set a property on exports then it will be set on module.exports because in JS, objects are pass by reference.
Conclusion is always use module.exports to avoid confusion.
Hope this helps. Happy coding :)

Also, one things that may help to understand:
math.js
this.add = function (a, b) {
return a + b;
};
client.js
var math = require('./math');
console.log(math.add(2,2); // 4;
Great, in this case:
console.log(this === module.exports); // true
console.log(this === exports); // true
console.log(module.exports === exports); // true
Thus, by default, "this" is actually equals to module.exports.
However, if you change your implementation to:
math.js
var add = function (a, b) {
return a + b;
};
module.exports = {
add: add
};
In this case, it will work fine, however, "this" is not equal to module.exports anymore, because a new object was created.
console.log(this === module.exports); // false
console.log(this === exports); // true
console.log(module.exports === exports); // false
And now, what will be returned by the require is what was defined inside the module.exports, not this or exports, anymore.
Another way to do it would be:
math.js
module.exports.add = function (a, b) {
return a + b;
};
Or:
math.js
exports.add = function (a, b) {
return a + b;
};

Rene's answer about the relationship between exports and module.exports is quite clear, it's all about javascript references. Just want to add that:
We see this in many node modules:
var app = exports = module.exports = {};
This will make sure that even if we changed module.exports, we can still use exports by making those two variables point to the same object.

node does something like this:
module.exports = exports = {}
module.exports and exports refer to same object.
This is done just for convenience.
so instead of writing something like this
module.exports.PI = 3.14
we can write
exports.PI = 3.14
so it is ok to add a property to exports but assigning it to a different object is not ok
exports.add = function(){
.
.
}
↑ this is OK and same as module.exports.add = function(){...}
exports = function(){
.
.
}
↑ this is not ok and and empty object will be returned as module.exports still refers to {} and exports refer to different object.

There are two difference between module.exports and exports
When export a single class, variable or function from one module to another module, we use the module.exports. But export to multiple variables or functions from one module to another, we use exports.
module.exports is the object reference that gets returned from the require() calls. But exports is not returned by require().
see more details with examples >> link

As all answers posted above are well explained, I want to add something which I faced today.
When you export something using exports then you have to use it with variable. Like,
File1.js
exports.a = 5;
In another file
File2.js
const A = require("./File1.js");
console.log(A.a);
and using module.exports
File1.js
module.exports.a = 5;
In File2.js
const A = require("./File1.js");
console.log(A.a);
and default module.exports
File1.js
module.exports = 5;
in File2.js
const A = require("./File2.js");
console.log(A);

myTest.js
module.exports.get = function () {};
exports.put = function () {};
console.log(module.exports)
// output: { get: [Function], put: [Function] }
exports and module.exports are the same and a reference to the same object. You can add properties by both ways as per your convenience.

You can think of exports as a shortcut to module.exports within a
given module. In fact, exports is just a variable that gets
initialized to the value of module.exports before the module is
evaluated. That value is a reference to an object (empty object in
this case). This means that exports holds a reference to the same
object referenced by module.exports. It also means that by assigning
another value to exports it's no longer bound to module.exports.
This explanation from MDN is the most clear to me.
Basically, there is one object in memory which is referenced by 2 variables - exports and module.exports.
exports.a = 23
equals
module.exports = {a:23}
But,
exports = {a:23}
does not equal
module.exports = {a:23}
When you assign a new object to exports variable directly, then that variable does not refer to module.exports anymore.

export.something Javascript

Here is a code snippet
body.user_id = userObj._id;
exports.inFCID(conn, obj.fcid, body, fcid, 0).then(function (r) {
exports.getUserById(conn, body.user_id).then(function (u) {
console.log("after getuserbyid", u);
Here I am sort of didn't understand why user have done something like
exports.inFCID(conn, obj.fcid, body, fcid, 0).then(function (r
of to be precise what does it do? I have previously encounter things like module.exports and export default statement ( export something statements) but this seems to be new.. Can someone explain me what this snippets would normally do? Ignoring what is inside those functions (inFCID) or what does export.something do in middle of the code..

It's expected that inFCID export is defined in this module as well:
exports.inFCID = function inFCID (...) {...};
When an export is defined as function expression, inFCID function is not available as inFCID but as exports.inFCID.
Referring exported functions as exports.inFCID inside module is a common recipe to improve testability in CommonJS modules; the same recipe won't work with ESM; a module needs to be separated when used with ES modules, as explained in this answer. Module exports can be spied or mocked outside the module:
const foo = require('foo');
...
spyOn(foo, 'inFCID');
foo.bar();
expect(foo.inFCID).toHaveBeenCalled();
This would be impossible if inFCID(...) was referred directly.

exports is a regular object.
If you have a code like this:
function test() {}
module.exports.test = test
Then nodejs will convert it into something like this:
function moduleInvocation(module, exports) {
function test() {}
module.exports.test = test
}
// a rough dummy code, to illustrat what node does on require
function require(moduleName) {
var module = { exports : {} };
//
// some code that loades `moduleName` and wraps it into `moduleInvocation`
//
moduleInvocation(module, module.exports)
return module;
}
So if someone writes exports.inFCID() then it is not different to:
var obj = {};
obj.inFCID = function() {}
obj.inFCID();
But it does not make any sense to write it that way except if you compose the exports of the module out of the content of some sub files.

Javascript Module Pattern Uncaught Reference Error

I've been following this tutoral, and when referencing Module as an argument in ModuleTwo, everything works fine until I comment out Module.
My understanding is that the double pipes || and empty object {} will create an empty object in place of Module if it's undefined, but instead I'm getting an error in the console.
var Module = (function () {
var privateMethod = function () {
// private
};
var someMethod = function () {
// public
};
var anotherMethod = function () {
// public
};
return {
someMethod: someMethod,
anotherMethod: anotherMethod
};
})();
var ModuleTwo = (function (Module) {
Module.extension = function () {
// another method!
};
return Module;
})(Module || {});

Basically, there’s an error in the tutorial. One way to make things work was suggested by rplantiko, but it might be easier to just write window.Module || {} instead of Module || {}.
How things work here:
Accessing a non-existent property of any object yields undefined. However, referencing a variable that hasn’t been declared yields ReferenceError (so your understanding was a little bit off there).
Browser puts all global variables as properties onto global window object. Module in the tutorial is a global variable, because it’s declared outside all functions, so you can access it via window.Module, which will not cause ReferenceError if undefined (per previous point).
It might be a good practice to explicitly assign to window any global variable you define (e.g., do window.Module = (function () { … if you intend to make Module global), but that’s arguable and out of scope of this discussion.

When you apply the function defining your 2nd module to (Module || {}), the symbol Module cannot be resolved if the Module hasn't been declared earlier, which always gives a JavaScript error. If you want the 2nd Module to be defined even in the absence of the first Module, try the following:
var ModuleTwo = (function(Module) {
...
})(typeof Module == 'object' ? Module : {} );

meaning of module.exports= function in node.js

I am taking first steps with node.js and obviously one of the first things i tried to do was exporting some data from a module, so i tried this simple case:
dummy.js:
var user = "rally";
module.exports = {
user:user
};
and than required it from a different file like this:
var dummy = require('./dummy.js');
console.log(dummy.user); // rally
So far so good, every thing works, but now i dived into code where there is this definition in the beginning of the module:
module.exports = function(passport,config, mongoose) {}
and i don't understand whats the meaning of it and how can i work with it.
just for trying to understand i defined some variables inside this abstract function but couldn't get their value from any other file.
any idea how can i export variables from module defined like this..? so for example i could require this module and get the "Dummy" variable and use it in a different file
module.exports = function(passport,config, mongoose) {
var dummy = "Dummy";
}

It works exactly the same as the first one does, only that it exports a function instead of an object.
The module that imports the module can then call that function:
var dummy = require('./dummy.js');
dummy();
any idea how can i export variables from module defined like this..?
Since functions are just objects, you can also assign properties to it:
module.exports = function(passport,config, mongoose) {}
module.exports.user = 'rally';
However I'd argue that this is less expected if a module directly exports a function. You are probably better off exporting the function as its own export:
exports.login = function(passport,config, mongoose) {}
exports.user = 'rally';

WHAT IS A MODULE?
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.
// dummy.js
var exports = module.exports = {};
The utility of dummy.js increases when its encapsulated code can be utilized in other files. This is achieved by using exports.
HOW ARE THEY INVOKED?
You could declare your functions outside of the module.exports block. Functions inside exports can be invoked exactly the same way as variables or any other object.
EXAMPLE
//dummy.js
var myVariable = "foo";
var myFunction = function(){
//some logic
};
module.exports{
myVariable : myVariable,
myFunction : myFunction,
myVariableTypeTwo : "bar",
myFunctionTypeTwo : function () {
//some logic
}
}
We can now access the publicly available methods of dummy.js as a property from any js file.
var dummy = require('./dummy.js');
dummy.myVariable; //foo
dummy.myFunction();
dummy.myVariableTypeTwo; //bar
dummy.myFunctionTypeTwo();
NOTE
In the code above, we could have replaced module.exports with exports and achieved the same result. If this seems confusing, remember that exports and module.exports reference the same object.

In Node.JS, how do I return an entire object from a separate .js file?

I am new to Node.js and trying to figure out how to request an object from a separate file (rather than just requesting a function) but everything I try--exports,module-exports,etc--is failing.
So, for example, if I have foo.js:
var methods = {
Foobar:{
getFoo: function(){return "foo!!";},
getBar: function(){return "bar!!";}
}
};
module.exports = methods;
And now I want to call a function within an object of foo.js from index.js:
var m = require('./foo');
function fooMain(){
return m.Foobar.getFoo();
};
How do I do this? I have tried all sorts of combinations of exports and module-exports but they seem to only work if I call a discrete function that is not part of an object.

You said that you tried exports, but your code doesn't show it. Anything that you want to be visible from outside your module must be assigned to (or otherwise be referable from) module.exports. In your case, where you have an object already, you can just assign it to module.exports:
var methods = {
...
};
// You must export the methods explicitly
module.exports = methods;
module.exports isn't magic, it's a normal object, and you can treat it as such. Meaning that you could have assigned your methods directly to it, as in:
module.exports.Foobar = {};
module.exports.Foobar.getFoo = function() { ... };
...
Or, as you probably know, you could event replace it with a function:
module.exports = function() { return "It's ALWAYS over 9000!!!!"; };
Only after exporting will you be able to use anything in another module.