When you install a package using npm, you can use the --save or --save-dev flag to specify how the package should be saved in your project's package.json file.

The --save flag adds the package to the dependencies field in your package.json file. This means that the package is required for your project to work, and it will be installed when someone runs npm install on your project.

The --save-dev flag adds the package to the devDependencies field in your package.json file. This means that the package is only needed for development, and it will not be installed when someone installs your project in production.

For example, if you are using a testing framework like Mocha, you might want to install it as a dev dependency like this:

npm install --save-dev mocha


This will install Mocha and add it to the devDependencies field in your package.json file.