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.