The package.json file is the NPM Configuration File; it basically contains a list of NPM packages that the developer want to be restored before the project starts. Those who already know what NPM is and how it works can skip to the next paragraph, while the others should definitely keep reading.

NPM (shortcode for Node Package Manager) started its life as the default package manager for the JavaScript runtime environment known as Node.js. During the latest years, though, it was also being used to host a number of independent JS projects, libraries, and frameworks of any kind, including Angular; eventually, it became the de facto package manager for JavaScript frameworks and tooling. If you never used it, you may easily think of it as the Nuget for the JavaScript world.

Although NPM is mostly a command-line tool, the easiest way to use it from Visual Studio is to properly configure a package.json file containing all the NPM packages we want to get, restore, and keep up to date later on. These packages get downloaded in the /node_modules/ folder within our project directory, which is hidden by default within Visual Studio; however, all the retrieved packages can be seen from the /Dependencies/npm/ virtual folder. As soon as we add, delete, or update the package.json file, Visual Studio will automatically update that folder accordingly.

In the Angular SPA template we've been using, the shipped package.json contains a huge amount of packages--all Angular packages plus a good bunch of dependencies, tools, and third-party utilities such as Karma (a great Test Runner for JavaScript/TypeScript).

Before moving ahead, let's take an additional look at our package.json file and try to get the most out of it. We can see how all the packages are listed within a standard JSON object entirely made of key-value pairs; the package name is the key, while the value is used to specify the version number. We can either input precise build numbers or use the standard npmJS syntax to specify auto-update rules bound to custom version ranges using the supported prefixes, such as the following: