Node.js Tutorial - Node.js Module System

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Node.js uses File Based Module System.

Each file is its own module.

Each file has access to the current module definition using the module variable.

The export of the current module is determined by the module.exports variable.

To import a module, use the globally available require function.

Example

The following code creates a function in file myData.js to reuse in various parts of our application.

To export the function from the file located in c:/intro/base/myData.js , we simply assign it to module.exports.

module.exports = function () {
   console.log('a function in file myData'); 
}; 

In order to use this function from a file c:/intro/base/myValue.js, import myData using the globally require function and store the returned value in a local variable.

var myData = require('./myData'); 
myData(); // logs out : "a function in file myData" 

module.exports

Each file in Node.js is a module.

The items to export from a module should be attached to the module.exports variable.

module.exports is defined to be a new empty object in every file.

module.exports = {} is implicitly present.

By default, every module exports an empty object, {}.

console.log(module.exports); // {} 

Exports Alias

We can export more than one variable from a module.

One way of achieving this is to create a new object literal and assign that to module.exports.

In c:/intro/exports/myData1.js we have

var a = function () {
    console.log('a called'); 
}; /*from   w ww.j  a va2s  .  c o m*/
var b = function () {
    console.log('b called'); 
}; 

module.exports = {
    a: a, 
    b: b 
}; 

In c:/intro/exports/myData2.js we have

module.exports.a = function () { 
    console.log('a called'); 
}; 

module.exports.b = function () { 
    console.log('b called'); 
}; 

Node.js helps us by creating an alias for module.exports called exports so instead of typing module.exports.something every time, you can simply use exports.something.

In c:/intro/exports/myData3.js we have

exports.a = function () { 
    console.log('a called'); 
}; 
exports.b = function () { 
    console.log('b called'); 
}; 

exports is just like any other JavaScript variable.

Node.js simply does exports = module.exports.

If we add something for example, myData to exports, that is exports.myData = 123, we are effectively doing module.exports.myData = 123 since JavaScript variables are references.

The following code shows that all of these methods are equivalent from consumption (import) point of view.

In c:/intro/exports/app.js we have

var myData1 = require('./myData1'); 
myData1.a(); // www .  j  av  a2s.  c o m
myData1.b(); 

var myData2 = require('./myData2'); 
myData2.a(); 
myData2.b(); 

var myData3 = require('./myData3'); 
myData3.a(); 
myData3.b(); 

Modules Best Practices

Do Not Use the .js Extension

It is better to do require('./myData') instead of require('./myData.js') even though both work fine for Node.js.

Relative Paths

When using file-based modules, you need to use relative paths (in other words, do require('./myData') instead of require('myData')).

Utilize exports

Try and use the exports alias when you want to export more than one thing.

The following code shows how to Create a Local Variable and Also Export

var myData = exports.myData = /* whatever you want to export as `myData` from this module */ ; 

Export an Entire Folder

If you have too many modules that go together that you keep importing into other files, try to avoid repeating the import.

var myData = require('../something/myData'); 
var myValue = require('../something/myValue'); 
var another = require('../something/another'); 
var third = require('../something/third'); 

Instead, create a single index.js in the something folder. In index.js, import all the modules once and then export them from this module.

In c:/yourFolder/index.js we have

exports.myData = require('./myData'); 
exports.myValue = require('./myValue'); 
exports.another = require('./another'); 
exports.third = require('./third'); 

Now you can simply import this index.js whenever you need all these things:

var something = require('../something/index'); 

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