Node js: Event Loop
To support the event-loop approach, Node supplies a set of “nonblocking” libraries. In essence, these are interfaces to things such as the filesystem or databases, which operate in an event-driven way. When you make a request to the filesystem, rather than requiring Node to wait for the hard drive to spin up and retrieve the file, the nonblocking interface simply notifies Node when it has access, in the same way that web browsers notify your code about an
onclickevent. This model simplifies access to slow resources in a scalable way that is intuitive to JavaScript programmers and easy to learn for everyone else.