1. Check expired timers

The first step in the event loop is checking for expired timers, and we do this in the self.check_expired_timers() function

    fn process_expired_timers(&mut self) {
        // Need an intermediate variable to please the borrowchecker
        let timers_to_remove = &mut self.timers_to_remove;

            .for_each(|(k, _)| timers_to_remove.push(*k));

        while let Some(key) = self.timers_to_remove.pop() {
            let callback_id = self.timers.remove(&key).unwrap();
            self.callbacks_to_run.push((callback_id, Js::Undefined));

    fn get_next_timer(&self) -> Option<i32> {
        self.timers.iter().nth(0).map(|(&instant, _)| {
            let mut time_to_next_timeout = instant - Instant::now();
            if time_to_next_timeout < Duration::new(0, 0) {
                time_to_next_timeout = Duration::new(0, 0);
            time_to_next_timeout.as_millis() as i32

The first thing to note here is that we check self.timers and to understand the rest of the syntax we'll have to look what kind of collection this is.

Now I chose a BTreeMap<Instant, usize> for this collection. The reason is that I want to have many Instant's chronologically. When I add a timer, I calculate at what instance it's supposed to be run and I add that to this collection.

BTrees are a very good data structure when you know that your keys will be ordered.

Choosing a BTreeMap here allows me to get a range range(..=Instant::now()) which is from the start of the map, up until or equal to the instant NOW.

Now I take every key in this range and add it to timers_to_remove, and the reason for this is that I found no good way to both get a range and remove the keys in one operation without allocating a small buffer every time. You can iterate over the range but due to the ownership rules you can't remove them at the same time, and we want to remove the timers that we're done with.

The event loop will run repeatedly, so avoiding any allocations inside the loop is smart. There is no need to have this overhead.

while let Some(key) = timers_to_remove.pop() {
    let callback_id = self.timers.remove(&key).unwrap();
    self.callbacks_to_run.push((callback_id, Js::Undefined));

The next step is to take every timer that has expired, remove the timer from our self.timers collection and get their callback_id.

As I explained in the previous chapter, this is an unique Id for this callback. What's important here is that we don't run the callback immediately. Node actually registers callbacks to be run on the next tick. An exception is the timers since they either have timed out or is a timer with a timeout of 0. In this case a timer will not wait for the next tick if it has timed out (or if it has a timeout of 0) but instead they will be invoked immediately as you'll see next.

Anyway, for now we add the callback ids to self.callbacks_to_run.

Before we continue, let's recap by looking what members of the Runtime struct we used here:

pub struct Runtime {
    pending_events: usize,
    callbacks_to_run: Vec<(usize, Js)>,
    timers: BTreeMap<Instant, usize>,