Explain the JavaScript event loop like I'm five_

There are two queues. Microtasks (Promise callbacks, queueMicrotask) drain completely before the next macrotask (setTimeout, setInterval, I/O callbacks) runs. This is why a resolved Promise callback always fires before a setTimeout(fn, 0).

Stickying the Philip Roberts JSConf EU talk as required watching for this thread. It's 26 minutes and will save you hours of confusion.

JavaScript is single-threaded. There is one call stack. Only one thing runs at a time. The event loop is the mechanism that makes it feel concurrent: it picks tasks off a queue and pushes them onto the stack when the stack is empty.

Philip Roberts' talk 'What the heck is the event loop anyway?' on YouTube and the Loupe visualiser at latentflip.com/loupe are the best resources I have found. Watching the stack, queue, and heap animate in real time makes it concrete.