dependencies vs devDependencies

Dependencies

Dependencies that your project needs to run, like a library that provides functions that you call from your code.

dependencies are installed on both:

• a directory that contains package.json
• npm i $package on any other directory

devDependencies

Dependencies you only need during development or releasing, like compilers that take your code and compile it into javascript, test frameworks or documentation generators.

devDependencies are installed on:

• a directory that contains package.json (unless given --production flag)
  • or if NODE_ENV=production environment variable is set
• NOT installed on npm install $package on any other directory
• are not installed transitively

From end user perspective, you normally don't want the development dependencies, so you just get what is needed to use the package (dependencies, not devDependencies).

If you want to develop, you would need additional packages needed for dependencies like tests.

What is transitive

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.

• we don't need to test B to test A, so B's testing dependencies can be left out.