Async programming in JS still runs your code in order, but it has features that allow:
We have seen event callbacks in Node's server listen and Express' route handlers.
router.get("/users", CALLBACK_FUNCTION); server.listen(port, CALLBACK_FUNCTION);
When client sends a "GET /users" request, call back this function
When server starts listening, call back this function
In this chapter, we'll look at the fundamentals of async programming: