As we stated in the introduction, Node.js came out of the idea that if we are writing code in the browser, then we should be able to run it on the server. Here, we have a single language for both contexts and we don't have to context switch when we work on either section.

Node.js can function in this way with a mixture of two libraries. These libraries are V8, which we should already be familiar with, and libuv, which we aren't currently familiar with. The libuv library gives us asynchronous I/O. Every OS has a different way of handling this I/O, so libuv gives us a nice C wrapper around all of these instances.

The libuv library queues up requests for I/O onto a stack of requests. Then, it farms them out to a certain amount of threads (Node.js utilizes four by default). Once the responses come back from these threads, libuv will put them on the response stack and alert V8 that the responses are ready to be consumed. Once V8 gets around to this alert, it will pull the value off and utilize it for its response to the request that we made. This is how the Node.js runtime is able to have asynchronous I/O and still maintain a single thread of execution (at least, that's how it looks to the user).

With this basic understanding, we should be able to start writing some basic scripts that handle various I/O operations and utilize one of the ideas that made Node.js special: the streaming system.