What is setTimeout doing when set to 0 milliseconds?

A few useful facts might help clarify what's happening:

1. JavaScript is single-threaded. Asynchronous callbacks are assigned to a message placed in a message queue.
2. When no code is currently executing, the event loop polls the message queue, requesting the next message in line to be processed (executed).
3. setTimeout adds a message (with the callback provided) to the end of this queue after the specified delay has elapsed.

(Note: this means the delay in a setTimeout call is not a sure thing; it is the minimum delay before the callback is executed. The actual time taken depends on how long it takes to process any messages ahead of it in the queue.)

So what happens if the delay is set to 0? A new message is added to the queue immediately, and will be processed when the currently executing code is finished and any previously-added messages have been processed.

What's happening in your code

When you invoke setTimeout…

setTimeout(function() { 
    console.log('AAA');
}, 0);

…a message gets added to the queue with the specified callback. The rest of your code…

for (i = 0; i < 1000; i++) {
    console.log('BBB'); 
}
// etc.

…continues executing synchronously. Once it has completely finished, the event loop polls the message queue for the next message and finds the one with your setTimeout callback, which is then processed (the callback is run).

The callback only ever gets executed after the currently executing code has finished, no matter how long that takes.

Further reading

For more details on the event loop, see:

• Concurrency model and event loop—MDN
• The JavaScript event loop: explained—Carbon Five