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Dylan Paulus
Dylan Paulus

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NPM: What are project dependencies?

Code dependencies are like Lego's. We're able to pull in other people's code; combining and stacking different packages together to fulfill our goals. Using dependencies greatly reduces the complexity of developing software. We can take advantage of the hard work someone has already done to solve a problem so we can continue to build the projects we want. A development pipeline can have multiple kinds of code dependencies:

  1. dependencies
  2. developer dependencies (devDependencies)
  3. peer dependencies (peerDependencies)

In JavaScript, we have a package.json file that holds metadata about our project. package.json can store things like our project name, the version of our project, and any dependencies our project has. Dependencies, devDependencies, and peerDependencies are properties that can be included in a package.json file.

    "dependencies": {
    "devDependencies": {
    "peerDependencies": {
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Production vs. Development

Depending on the instance where code will be used changes the type of dependency a package is. There are packages that our users will need to run our code. A user is someone not directly working in our code-base. This could mean a person interacting with an application we wrote, or a developer writing a completely separate library. In other words, this is a production environment. Alternatively, there are packages that a developer or system only needs while working in our code. For example linters, testing frameworks, build tools, etc. Packages that a user won't need, but a developer or build system will need.


Dependencies are packages our project uses in production. These get included with our code and are vital for making our application run. Whenever we install a dependency the package and any of its dependencies get downloaded onto our local hard drive. The more dependencies we add, the bigger our production code becomes. This is because each new dependency gets included in the production build of our code. Evaluate adding new dependencies unless they're needed!

Dependencies are installed using npm install X or yarn add X

Examples of dependencies: React, stylized-components, jQuery

Dev Dependencies

Packages needed in development, or while developing our code, are considered dev dependencies. These are programs, libraries, and tools that assist in our development workflow. Dev dependencies also get downloaded to your local hard drive when installed, but the user will never see these dependencies. So adding a lot of dev dependencies only affects the initial yarn or npm install completion time.

Dev Dependencies are installed using npm install --save-dev X or yarn add --dev X

Examples of Dev Dependencies: Jest, ESLint, Webpack

Peer Dependencies

Peer dependencies are similar to dependencies except for a few key features. First, when installing a peer dependency it doesn't get added to your node_modules/ directory on your local hard drive. Why is that? Well, peer dependencies are dependencies that are needed in production, but we expect the user of our code to provide the package. The package doesn't get included in our code. This is to reduce including multiples of the same dependency in production. If every React library included a version of React as a dependency, then in production our users would download React multiple times. Peer dependencies are a tool for library owners to optimize their project size.

Peer Dependencies are installed using yarn add --peer X

Examples of Peer Dependencies: React, Bootstrap


I recently released a course, Creating React Libraries from Scratch, where we walk through deploying a React library from yarn init to yarn publish. Creating React Libraries from Scratch includes content just like this and more!

To learn more click the image below!

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