How to install only “devDependencies” using npm

Check the NPM docs for install:

With the --production flag (or when the NODE_ENV environment variable is set to production), npm will not install modules listed in devDependencies.

The --only={prod[uction]|dev[elopment]} argument will cause either only devDependencies or only non-devDependencies to be installed regardless of the NODE_ENV.

Have you tried the following?

npm install --only=dev