Why use JavaScript on the server side?

There are many server-side languages: Java, PHP, Ruby, Go, and our friend Python, just to name a few. So, why would we want to use JavaScript as a server-side language? One answer is to reduce context switching. In theory, the same developer can write both the front- and backend of a web application with a minimum of mental changes. The research behind the cost of switching programming languages is light so far and tends to be highly anecdotal, but some studies have shown that the cognitive overhead of switching from one task to another and back again reduces productivity and increases the length of time it takes to complete a task. By extension, switching from JavaScript to Python requires a few mental gymnastics. Of course, with practice, this mental overhead becomes unimportant (think of a translator who can in real time listen to one language and translate this to a different language). However, with the speed at which technology changes, reaching...