Using npm Packages with Node.js

Now that we've installed a package, let's learn how to use it in our Node.js application.

The Node.js require function

If you recall from the beginning of this class, the Node.js module loader and npm co-evolved. That is to say, they were written with each other in mind.

Node.js' module loader uses a function called require that anticipates a node_modules directory. When you want to use a package you can type:

//index.js

require('mod-a');

When Node.js sees this code, it will look for a node_modules directory, and then it will look for a mod-a directory inside. If it finds it, it will let you use the code from mod-a in your application. If it doesn't you'll get an error:

module.js:340
    throw err;
    ^

Error: Cannot find module 'mod-a'
...

When you see this error, it means that Node.js could not find your package. This mean you'll need to install it using the npm install command.