How does JavaScript work?

JavaScript is run by an engine, e.g. V8 in Chrome and Node.js. Synchronous code runs on a single main thread and uses the call stack.

If you call a function:

function first() {
  second();
}

function second() {
  console.log("Hello");
}

first();

the stack looks roughly like this:

first()
  ↓
second()
  ↓
console.log()

When they finish, functions are popped off the stack.

JavaScript can also handle asynchronous operations, e.g.:

setTimeout(...)
fetch(...)

They don't block the whole program. The browser or Node.js environment handles them, and when they finish, their callback can come back for execution through the event loop.

To be precise:
JavaScript is single-threaded as far as running JS code goes, but the whole environment doesn't have to run on one physical thread.