Angular 4 also depends on a number of other JavaScript libraries to function; some of those libraries are peerDependencies within Angular's own dependencies. That means that they must be fulfilled by your application in order for Angular to function. Some of these dependencies, such as TypeScript, were actually provided in the NPM install command; however, some of them, such as Zone.js, are not. These peerDependencies can vary even between minor version differences of Angular, so if you do see one of these warnings, your best bet is to manually update your package.json dependency for that library and try again:

Now that our Angular application is upgraded to Angular 4, we will move on to using Angular 4 for the rest of this book's Angular content. From now on, when I use the term Angular, it is specifically in reference to Angular 4. This conforms with Google’s own usage of the name, always using Angular generically to be the latest major SemVer version of the Angular framework.