When we start a new javascript project and we count on adding a minimal level of complexity to it, we will eventually need to make use of external libraries if we want to avoid writing every feature totally from scratch. For that matter, you can choose to take three approaches:

  • Download the library code bundle and adapt it to your development and deployment process. This is easy if the library is very small, you don't care about updating it's version, and you know very specifically what you want from it.
  • Use a Content Delivery Network to access a remote copy of the library hosted in the cloud.
  • Use a tool like NPM, Bower, or the new kid in town Yarn. This are package managers: pieces of software that allow the developer manage the projet's dependencies by fetching them from a central repository, specify each library's desired version and execute scripts for different steps of the development process.

When you are working on a project and decide to use npm as it's package manager, it will use a file named package.json in the root of the project's directory. This file contains the information about the dependencies on external libraries it has, along with other useful information. It sits as the point of reference that npm will use when fetching and updating your dependencies. More information about the package.json file [here](link to package.json explanation).

How does it work.

Very briefly explained, npm will read the contents of your package.json file and will enumerate the libraries your current project depends on. It will then fetch each library in a recursive manner, namely, each one of those libraries may contain themselves a package.json file enumerating the dependencies that library relies upon, so it'll fetch them again, and so on, and so on.

Dependencies are expressed in a hash with a pair of

  "package_name_1": "version range",
  "package_name_2": "version range",

per each of them. The version range can be a string indicating the version range to fetch from the central repository, a tarball or a git url pointing to a specific tag or commit.

There are different ways inside the package.json file to indicate the nature of the dependencies: dependencies, devDependencies and peerDependencies.
You can also find bundledDependencies and optionalDependencies, but I do not see them used that often.


These are the libraries your project directly depends on in order to properly execute. There is a direct dependency from the correct functioning of your application to the fetching of this packages. After compiling and minifying your javascript, the final bundle must include these libraries code to work.


They represent the packages needed during the development of your project, such as tools and frameworks, that are not needed in the final product. An example for a project developed with the Ember javascript framework, could be:

"devDependencies": {
  "ember-cli": "2.11.1",
  "ember-cli-sass": "^5.3.1",
  • ember-cli is the command line tool that helps the creation of blueprints for the project, run scripts, etc.. much as the rails cli tool.
  • ember-cli-sass contains files to help translate the sass code into css code.

As you can see, a final user of your product is not interested to have this auxiliary tools in the final bundle.


peerDependencies are used to avoid name clashes and application crashes when a package depends on a library's version that is not the same as the specified one in the project's package.json. For example:

  "name": "test_peer",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  "author": "",
  "license": "ISC",
  "dependencies": {
    "grunt-webpack": "^3.0.0",
    "webpack": "^1.0.0"

I specify version "1" of webpack and also install grunt-webpack. grunt-webpack states a different webpack version in it's peerDependencies hash:

"peerDependencies": {
  "webpack": "^2.1.0-beta || ^2.2.0-rc || 2 || 3"

The result of npm install this project is:

test_peer@1.0.0 /home/esteban/gits/test/test_peer
├─┬ grunt-webpack@3.0.0
│ ├─┬ deep-for-each@1.0.6
│ │ └─┬ is-plain-object@2.0.3
│ │   └── isobject@3.0.0
│ └── lodash@4.17.4
└─┬ UNMET PEER DEPENDENCY webpack@1.0.0

... etc ...

This means that while in my project I downloaded version 1 of webpack, grunt-webpack depends on another set of versions to safely execute, so the package manager will intercede for us and warn us that this setup may break the program.


They can also be written as bundleDependencies. They enumerate the dependencies that must be included in your project just as regular dependencies. They will also be included in the tarball generated as the result of executing npm pack.

They are used for example if you want to include third party libraries that are not part of the npm registry, or distribute your project as a module.


Just like regular dependencies, but builds don't fail if the dependencies are not met.

That's it, I hope it was a little bit more clarifying for readers!