30 minutes

• How to require node modules in a project.
• How to require a JSON file as a module
• How to require a core module
• How to require from the node_modules folder
• How to export methods in a module

In node, modules work by exporting methods or values. If this were visualized in JavaScript it might be similar to this:

var exports = {};
(function() {
  var a = 10;
  exports.foo = a * 10;
})();

console.log(exports.a);   // undefined
console.log(exports.foo); // 100

Notice the (function () {/* module code here*/} )();. This provides a new scope for the module to prevent pollution of the global object.

If written into a node module, this would be written as:

var a = 10;
exports.foo = a * 10;

In this lab we'll require() a file into our project.

First create a folder called /planets.
Inside of /planets, create a file called circle.js with the following contents:

var PI = Math.PI;

exports.area = function (r) {
  return PI * r * r;
};

exports.circumference = function (r) {
  return 2 * PI * r;
};

The module circle.js has exported the functions area() and circumference(). To export an object, add to the special exports object. Variables local to the module will be private. In this example the variable PI is private to circle.js.

Next add another file to /planets called earth.js with the following contents:

var circle = require('./circle');
var radius = 6378.1;
console.log( 'The area of the planet earth is ' + circle.area(radius) + ' km2');

Now, run the app by typing node earth from within the planets directory. You should see the following:

The area of the planet earth is 127800490.57763624 km2

A JSON file can be included in a project by doing a require('./data.json'). This is because .json files are parsed as JSON text files. Let's modify the previous example by storing the radius of all the planets in a JSON file.

In the planets directory, create a file called planets.json with the following contents:

{
  "mercury": 2440,
  "venus":  6051,
  "earth":  6378,
  "mars":   3397,
  "jupiter": 71492,
  "saturn": 60268,
  "uranus": 25559,
  "neptune": 24764
}

View the planets.json file by firing up a node shell (node without arguments) from within the /planets directory and typing the following:

var planets = require('./planets.json');
planets.neptune;

Try to output the radius of some other planets.

Once you are comfortable with planets.json object, update the earth.js example by importing planets.json into earth.js.

var planets = require('./planets.json');

Finally, update earth.js to use the circumference stored in planets.json.

Node has several modules compiled into the binary. The core modules are defined in node's source in the lib/ folder. Core modules are always preferentially loaded if their identifier is passed to require(). For instance, require('http') will always return the built in HTTP module, even if there is a file by that name.

Let's use the core module assert to test circle.js. In the planets/ folder, create a file called test.js.
In test.js, require the assert module and circle.js to test the circle.area and circle.circumference methods.

var assert = require('assert');
var circle = require('./circle');

Next write a couple tests using the documentation for assert

Modules can be installed using the node package manager, npm, to be included into your application. For the next example, let's print out the planets in order of size. To do this, we'll include the underscore.js module. First we need to install underscore by executing the following command from within the planets/ directory.

npm install underscore

Now create a file called size.js and add the following:

var sortBy = require('underscore').sortBy,
    keys = require('underscore').keys,
    planets = require('./planets.json');

planets = sortBy(keys(planets), function(k) { return planets[k]; })

console.log(planets);

See underscore.js documentation.