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Introduction to npm

npm is the standard package manager for Node.js. In September 2022 over 2.1 million packages were reported being listed in the npm registry, making it the biggest single language code repository on Earth, and you can be sure there is a package for (almost!) everything. It started as a way to download and manage dependencies of Node.js packages, but it has since become a tool used also in frontend JavaScript.
Yarn and pnpm are alternatives to the npm CLI. You can check them out as well.

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Packages

npm installs, updates and manages downloads of dependencies of your project. Dependencies are pre-built pieces of code, such as libraries and packages, that your Node.js application needs to work.

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Installing all dependencies

If a project has a package.json file, by running 
npm install
it will install everything the project needs, in the node_modules folder, creating it if it’s not existing already.

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Installing a single package

You can also install a specific package by running 
npm install <package-name>
Furthermore, since npm 5, this command adds <package-name> to the package.json file dependencies. Before version 5, you needed to add the flag --save. Often you’ll see more flags added to this command:
  • --save-dev installs and adds the entry to the package.json file devDependencies
  • --no-save installs but does not add the entry to the package.json file dependencies
  • --save-optional installs and adds the entry to the package.json file optionalDependencies
  • --no-optional will prevent optional dependencies from being installed
Shorthands of the flags can also be used:
  • -S: --save
  • -D: --save-dev
  • -O: --save-optional
The difference between devDependencies and dependencies is that the former contains development tools, like a testing library, while the latter is bundled with the app in production. As for the optionalDependencies the difference is that build failure of the dependency will not cause installation to fail. But it is your program’s responsibility to handle the lack of the dependency. Read more about optional dependencies.

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Updating packages

Updating is also made easy, by running 
npm update
npm will check all packages for a newer version that satisfies your versioning constraints. You can specify a single package to update as well: 
npm update <package-name>

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Versioning

In addition to plain downloads, npm also manages versioning, so you can specify any specific version of a package, or require a version higher or lower than what you need. Many times you’ll find that a library is only compatible with a major release of another library. Or a bug in the latest release of a lib, still unfixed, is causing an issue. Specifying an explicit version of a library also helps to keep everyone on the same exact version of a package, so that the whole team runs the same version until the package.json file is updated. In all those cases, versioning helps a lot, and npm follows the semantic versioning (semver) standard. You can install a specific version of a package, by running 
npm install <package-name>@<version>

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Running tasks

The package.json file supports a format for specifying command line tasks that can be run by using 
npm run <task-name>
For example: 
{
  "scripts": {
    "start-dev": "node lib/server-development",
    "start": "node lib/server-production"
  }
}
It’s very common to use this feature to run Webpack: 
{
  "scripts": {
    "watch": "webpack --watch --progress --colors --config webpack.conf.js",
    "dev": "webpack --progress --colors --config webpack.conf.js",
    "prod": "NODE_ENV=production webpack -p --config webpack.conf.js"
  }
}
So instead of typing those long commands, which are easy to forget or mistype, you can run 
npm run watch
npm run dev
npm run prod

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