Introduction

[Javascript is single-threaded and synchronous](https://stackoverflow.com/questions/2035645/when-is-javascript-synchronous#:~:text=JavaScript is always synchronous and single-threaded.&text=JavaScript is only asynchronous in,the callback will run synchronously.), but it allows you to write async code.

Async programming in JS still runs your code in order, but it has features that allow:

• non-blocking - an external operation like I/O, data fetch or file operation doesn't block execution. i.e. code continues execution to next line
• event-driven - events can trigger code outside of the normal program flow

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:

• writing non-blocking code
• transforming code from callbacks to promises to async-await
• establishing order and avoiding "hells"
• using Node and Express async APIs

Terminologies

• Caller or sender - client code that calls the async function
• Receiver - service code that provides async function