I’ve found that most explanations about how Node.js handles asynchronous control flow are difficult for me to understand because those explanations assume either 1) a deep knowledge of computer science or 2) the reader is only interested in Node.js, not the foundational pieces it is built on.

This post is my attempt to fill this gap.

Concurrency In General

Concurrency is everywhere in the modern programming landscape:

• A web site might have to handle many simultaneous users.
• A set of APIs that need to coordinate transactions might be distributed across many computers in a cloud computing environment.
• An IDE might compile code in the background as a developer writes their code. …