Install packages required by your application in production:
npm install [package name]
Install packages that are only needed for local development and testing:
npm install [package name] --save-dev
NPM cheat sheet:
https://kapeli.com/cheat_sheets/npm.docset/Contents/Resources/Documents/index
Specifying dependencies and devDependencies in a package.json fileTo specify the packages your project depends on, you must list them as “dependencies” or “devDependencies” in your package’s package.json file. When you (or another user) run npm install, npm will download dependencies and devDependencies that are listed in package.json that meet the semantic version requirements listed for each. To see which versions of a package will be installed, use the semver calculator.”dependencies”: Packages required by your application in production.”devDependencies”: Packages that are only needed for local development and testing.
To specify the packages your project depends on, you must list them as "dependencies" or "devDependencies" in your package’s package.json file. When you (or another user) run npm install, npm will download dependencies and devDependencies that are listed in package.json that meet the semantic version requirements listed for each. To see which versions of a package will be installed, use the semver calculator.
"dependencies": Packages required by your application in production."devDependencies": Packages that are only needed for local development and testing.
Source: Specifying dependencies and devDependencies in a package.json file | npm Documentation