Node.js Tutorial - Node.js Packages

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Node.js comes with its own package management system called Node Package Manager (NPM).

There are three kinds of Node.js modules: file-based modules, core modules, and external node_modules.

If the module name passed into the require function is prefixed with './' or '../' or '/', then it is assumed to be a file-based module and the file is loaded.

Otherwise, we look for core modules with the same name, for example, utilif the call was require('util').

If no core module matching this name is found, we look for a node_module called util.

Package Locations

Packages are stored in a subdirectory named node_modules within your current directory.

To determine the location, use the command npm root.

To view all the installed modules using the npm ls command.

After installing the commander module, you can verify it using npm ls.

$ npm ls
/npm-test
 |
 +--commander@1.3.2
     |
     +--keypress@0.1.0

The code above shows the installed commander module. The tree structure indicates that commander depends on the keypress module.

npm can recognize its dependency and automatically installs any required modules.

You can see the installed modules by browsing the node_modules subdirectory.

Scanning for node_modules

If a file /home/myUser/project/myData.js has a require call require('myModule'), Node.js scans the file system for node_modules in the following order.

The first myModule.js that is found is returned.

/home/myUser/project/node_modules/myModule.js 
/home/myUser/node_modules/myModule.js 
/home/node_modules/myModule.js 
/node_modules/myModule.js 

Node.js looks for node_modules/myModule.js in the current folder followed by every parent folder until it reaches the root of the file system tree for the current file or until a myModule.js is found.

The following code in c:/hello/myData.js loads a module node_modules/myModule.js.

var myModule = require('myModule'); 
myModule(); // hello node_modules! 

File under c:/hello/node_modules/myModule.js

module.exports = function () {
    console.log('hello node_modules!'); 
} 

module myModule.js looks exactly the same if we were using file-based modules.

The only difference between file-based modules and node_modules is the way in which the file system is scanned to load the JavaScript file. All other behavior is the same.

Folder-Based Modules

Node.js will look for an index.js file in that folder and return that as the module file.

The following code show to do Implicit Loading of index.js from a Folder.

File under bar/bar1.js

module.exports = function () {
    console.log('bar1 was called'); 
} 

File under bar/bar2.js

module.exports = function () {
   console.log('bar2 was called'); 
} 

File under bar/index.js

exports.bar1 = require('./bar1'); 
exports.bar2 = require('./bar2'); 

Here is the File of myData.js which would use the bar module.

var bar = require('./bar'); 
bar.bar1(); 
bar.bar2(); 

Note

The following code shows how to change the folder based module to node_modules.

We can change the require call from require('./bar') to require('bar').

// node_modules/bar/bar1.js 
module.exports = function () {/*from  w  ww  . j  av  a  2 s . co  m*/
   console.log('bar1 was called'); 
} 
// node_modules/bar/bar2.js 
module.exports = function () {
   console.log('bar2 was called'); 
} 

// node_modules/bar/index.js 
exports.bar1 = require('./bar1'); 
exports.bar2 = require('./bar2'); 

// myData.js 
var bar = require('bar'); // look for a node_modules module named bar 
bar.bar1(); 
bar.bar2(); 

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