What's the difference between dependencies, devDependencies, and peerDependencies in NPM package.json file?

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• dependencies are installed on both:
  • npm install from a directory that contains package.json
  • npm install $package on any other directory
• devDependencies are:
  • also installed on npm install on a directory that contains package.json, unless you pass the --production flag, or if the NODE_ENV=production environment variable is set
  • not installed on npm install "$package" on any other directory, unless you give it the --dev option.
  • are not installed transitively.
• peerDependencies:
  • before 3.0: are always installed if missing, and raise an error if multiple incompatible versions of the dependency would be used by different dependencies.
  • expected to start on 3.0 (untested): give a warning if missing on npm install, and you have to solve the dependency yourself manually. When running, if the dependency is missing, you get an error (mentioned by @nextgentech) This explains it nicely: https://flaviocopes.com/npm-peer-dependencies/
  • in version 7 peerDependencies are automatically installed unless an upstream dependency conflict is present that cannot be automatically resolved
• Transitivity:
  • dependencies are installed transitively: if A requires B, and B requires C, then C gets installed, otherwise, B could not work, and neither would A.
  • devDependencies is not installed transitively. E.g. we don’t need to test B to test A, so B’s testing dependencies can be left out.