How do you reinstall an app's dependencies using npm?

The easiest way that I can see is delete node_modules folder and execute npm install.

The right way is to execute npm update. It's a really powerful command, it updates the missing packages and also checks if a newer version of package already installed can be used.

Read Intro to NPM to understand what you can do with npm.

Most of the time I use the following command to achieve a complete reinstall of all the node modules (be sure you are in the project folder).

rm -rf node_modules && npm install

You can also run npm cache clean after removing the node_modules folder to be sure there aren't any cached dependencies.