Code dependencies are like Legos. 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: dependencies developer dependencies (devDependencies) peer dependencies (peerDependencies) In JavaScript, we have a file that holds metadata about our project. 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 file. package.json package.json package.json {
    "dependencies": {
        ...
    },
    "devDependencies": {
        ...
    },
    "peerDependencies": {
        ...
    }
} 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 Dependencies are packages our project uses in . 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! production Dependencies are installed using or npm install X yarn add X Examples of dependencies: React, stylized-components, jQuery Dev Dependencies Packages needed in , 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 or completion time. development yarn npm install Dev Dependencies are installed using or npm install --save-dev X 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 directory on your local hard drive. Why is that? Well, peer dependencies are dependencies that are needed in , 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 multiples of the same dependency in . If every React library included a version of React as a dependency, then in our users would download React multiple times. Peer dependencies are a tool for library owners to optimize their project size. node_modules/ production production production Peer Dependencies are installed using yarn add --peer X Examples of Peer Dependencies: React, Bootstrap Conclusion I recently released a course, Creating React Libraries from Scratch, where we walk through deploying a React library from to . Creating React Libraries from Scratch includes content just like this and more! yarn init yarn publish Also Published here

Understanding Project Dependencies Through the NPM Lens

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