When working with single page web applications (SPA), chances are you’re going to be using a router or some kind of parent and child component structure. In these scenarios, the parent components probably hold their own functions and variables that may at some point in time need to be accessed by the child component. Take for example an application that has authentication. The parent component might manage the authentication state while the child component might be a login form, secure content, or something else. When logging in, you might need to change the authentication state of the parent. This is just one of many possible examples where the child and parent need to interact.