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README.md

Lesson 7: Modules

We can keep code organized and packaged for easy resue.

When creating an application, you often reach a point where putting all of your code in a single file becomes unwieldy. When this happends, the conventional approach is to take a file containing a lot of code and try to organize it by grouping related logic and moving it into separate files.

We've seen only built-in modules

var http = require('http');
var fs = require('fs');

How does require return the libraries?

How does it find these files?

Requring Modules

Java or Python use the import function to load other libraries, while PHP and Ruby use require. Node.js implements the CommonJS interface for modules. In Node.js you can also load other depencies using the require keyword.

  • Look in the same directory
var some_module = require('./some_module');
  • Look in the parent directory
var some_module = require('../some_module');
  • Look in the root directory
var some_module = require('/Users/dale/some_module');
  • Look in the node_modules directory
var some_module = require('some_module');

Creating Modules

Modules can either be single files or directories containing one or more files.

If a module is a directory, the file in the module directory that will be evaluated is normally named index.js.

  • ./my_module.js
  • ./my_module/index.js

Exporting Modules

Node modules allow you to select what functions and variables from the included file are exposed.

  • Expose a single function or variable by setting the property module.exports.
  • Expose multiple functions or varaiables by setting the properties of exports.

Export Multiple Functions

  • currency-functions.js
var canadianDollar = 0.91

function roundTwoDecimals(amount) {
	return Math.round(amount * 100) / 100;
}

var canadianToUs = function(canadian) {
	return roundTwoDecimals(canadian * canadianDollar);
};

var usToCanadian = function(us) {
	return roundTwoDecimals(us / canadianDollar);
};

exports.canadianToUs = canadianToUs;
exports.usToCanadian = usToCanadian;
  • test-currency-functions.js
var currency = require('./currency-functions');

console.log('50 Canadian dollars equals this amount of US dollars:');
console.log(currency.canadianToUs(50));

console.log('30 US dollars equals this amount of Canadian dollars:');
console.log(currency.usToCanadian(30));

Export a Single Class

  • currency-class.js
var Currency = function(canadianDollar) {
	this.canadianDollar = canadianDollar;
};

Currency.prototype.roundTwoDecimals = function(amount) {
	return Math.round(amount * 100) / 100;
};

Currency.prototype.canadianToUs = function(canadian) {
	return this.roundTwoDecimals(canadian * this.canadianDollar);
}

Currency.prototype.usToCanadian = function(us) {
	return this.roundTwoDecimals(us / this.canadianDollar);
}

module.exports = Currency;
  • test-currency-class.js
var Currency = require('./currency-class');

var currency = new Currency(0.91);

console.log('50 Canadian dollars equals this amount of US dollars:');
console.log(currency.canadianToUs(50));

console.log('30 US dollars equals this amount of Canadian dollars:');
console.log(currency.usToCanadian(30));

Modularizing current application

Before

var rectangle = {
	perimeter: function (length, width) {
    return 2 * (length + width);
	},
	area: function (length, width) {
    return length * width;
	}
};

function solveRectangle(length, width) {
  console.log("Solving for rectangle with length = " + length + " and width = " + width);

  if (length < 0 || width < 0) {
    console.log("Rectangle dimensions should be greater than zero");
  } else {
  	console.log("The area:", rectangle.area(length, width));
  	console.log("The perimeter:" + rectangle.perimeter(length, width));
  }
}

After

var rectangle = {
	perimeter: function (length, width) {
    return 2 * (length + width);
	},
	area: function (length, width) {
    return length * width;
	}
};

module.exports = rectangle;
var perimeter = function (length, width) {
  return 2 * (length + width);
};

var area = function (length, width) {
  return length * width;
};

exports.perimeter = perimeter;
exports.area = area;
var rectangle = require('./rectangle-1');
// var rectangle = require('./rectangle-2');

function solveRectangle(length, width) {
  console.log("Solving for rectangle with length = " + length + " and width = " + width);

  if (length < 0 || width < 0) {
    console.log("Rectangle dimensions should be greater than zero");
  } else {
  	console.log("The area:", rectangle.area(length, width));
  	console.log("The perimeter:" + rectangle.perimeter(length, width));
  }
}

NPM (Node Package Manager)

The Node Package Manager is a utility that comes bundled with Node.js.

It allows you to easily install third-party packages and globally publish any Node.js packages you yourself create.

$ npm --version
$ npm help

Searching for npm packages

Installing NPM packages

Install a package into your node_modules directory using npm install <package>:

$ npm install lodash
$ ls node_modules
lodash

Uninstalling NPM packages

Remove a package from your node_modules directory using npm uninstall <package>:

$ npm uninstall lodash
$ ls node_modules

Installing/Uninstalling packages globally

To download packages globally, you simply use the command npm install -g <package>, e.g.:

$ npm install -g standard
$ npm uninstall -g standard

Using a package.json

A package.json file is also known as a package descriptor file.

This file is always placed in an application's root directory and specify the name of your application, the version and a description of what the application does as well as the application's dependencies.

A package.json file affords you a lot of great things:

  • It serves as documentation for what packages your project depends on.
  • It allows you to specify the versions of a package that your project can use using semantic versioning rules.
  • Makes your build reproducible which means that its way easier to share with other developers.

Requirements

As a bare minimum, a package.json must have:

  • "name"
    • all lowercase
    • one word, no spaces
    • dashes and underscores allowed
  • "version"
    • in the form of x.x.x
    • follows semver spec
    • For example:
{
  "name": "my-awesome-package",
  "version": "1.0.0"
}

Creating a package.json

To create a package.json run:

> npm init

This will initiate a command line questionnaire that will conclude with the creation of a package.json in the directory you initiated the command.

> npm init --yes

This will ask no questions, and instead will generate a default package.json using information extracted from the current directory.

Specifying Packages

To specify the packages your project depends on, you need to list the packages you'd like to use in your package.json file. There are 2 types of packages you can list:

  • "dependencies": these packages are required by your application in production
  • "devDependencies": these packages are only needed for development and testing

Manually editing your package.json

The project below uses any version of the package lodash that matches major version 4 in production, and requires any version of the package nodemon that matches major version 1, but only for development:

{
  "name": "my_package",
  "version": "1.0.0",
  "dependencies": {
		"lodash": "^4.0.0"
  },
  "devDependencies" : {
		"nodemon": "^1.0.0"
  }
}

NPM reads dependencies from your package.json file and install each of them with a single command.

$ npm install
$ ls node_modules/
lodash nodemon

There should be a newly created node_modules directory which contains your application's dependencies.

The --save and --save-dev install flags

The easier (and more awesome) way to add dependencies to your package.json is to do so from the command line, flagging the npm install command with either --save or --save-dev, depending on how you'd like to use that dependency.

To add an entry to your package.json's dependencies:

npm install <package_name> --save

To add an entry to your package.json's devDependencies:

npm install <package_name> --save-dev

Run NPM scripts

npm run <script> is used by the test, start or any other commands.

"scripts": {
		"test": "echo \"Error: no test specified\" && exit 1",
    "start": "node server.js",
    "dev": "nodemon server.js"
  }

Publishing NPM packages

https://docs.npmjs.com/getting-started/publishing-npm-packages

Challenges

  • Highfive
  • LogLevel
  • Installing Local Modules : Practice using NPM by installing the npm module lodash using the npm command.
  • Dependency

Reference