As we mentioned earlier, the npm install command by itself installs the packages listed in the dependencies section of package.json. This is easy and convenient. Simply by listing all the dependencies, it's quick and easy to install the dependencies required for using the package. What happens is npm looks in package.json for the dependencies or devDependencies field, and it will automatically install the mentioned packages.

You can manage the dependencies manually by editing package.json. Or you can use npm to assist you with editing the dependencies. You can add a new dependency like so:

$ npm install akasharender --save  

In response, npm will add a dependencies tag to package.json:

"dependencies": { 
    "akasharender": "^0.6.15" 
} 

Now, when your application is installed, npm will automatically also install that package along with any dependencies listed by that package.

The devDependencies are modules used during development. That field is initialized the same as above, but with the --save-dev flag.

By default, when an npm install is run, modules listed in both dependencies and devDependencies are installed. Of course, the purpose for having two lists is to not install the devDependencies in some cases:

$ npm install --production  

This installs only the modules listed in dependencies and none of the devDependencies modules.

In the Twelve-Factor application model, it's suggested that we explicitly identify the dependencies required by the application. This way we can reliably build our application, knowing that we've tested against a specific set of dependencies that we've carefully identified. By installing exactly the dependencies against which the application has been tested, we have more confidence in the application. On the Node.js platform, npm gives us this dependencies section, including a flexible mechanism to declare compatible package versions by their version number.