When you install an npm package using npm install <package-name>
, you are installing it as a dependency.
The package is automatically listed in the package.json file, under the dependencies
list (as of npm 5: before you had to manually specify --save
).
When you add the -D
flag, or --save-dev
, you are installing it as a development dependency, which adds it to the devDependencies
list.
Development dependencies are intended as development-only packages, that are unneeded in production. For example testing packages, webpack or Babel.
When you go in production, if you type npm install
and the folder contains a package.json
file, they are installed, as npm assumes this is a development deploy.
You need to set the --production
flag (npm install --production
) to avoid installing those development dependencies.