Difference Between NPM Dependencies Types

ref: stackoverflow

Summary of important behavior differences:

• 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
  • not installed on npm install $package on any other directory, unless you give it the --dev option.
  • are not installed transitively.
• peerDependencies are:
  • 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 starting 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.
• 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 are not installed transitively. E.g. we don't need to test B to test A, so B's testing dependencies can be left out.