The npm install command will install the devDependencies along other dependencies when run inside a package directory, in a development environment (the default).
In version 8.x and above use --omit=dev flag to install only regular dependencies:
npm install --omit=dev
This will install only dependencies, and not devDependencies, regardless of the value of the NODE_ENV environment variable.
If you use 6.x or an earlier version, you need to use the --only=prod flag instead.
Note:
Before v3.3.0 of npm (2015-08-13), the option was called --production, i.e.
npm install --production
You may also need --no-optional flag.