node.export中的module.exports = function的含义

Question

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";
}

Answer 1

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';

Answer 2

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.