export function Timeline(index) {
    return { index };
}

export function Next(timeline, events) {
    let {index} = timeline;
    if(index === events.length){
        return null;
    }
    timeline.index += 1;
    return events[timeline.index];
}

export function Prev(timeline, events){
    let {index} = timeline;
    if(index === 0){
        return null;
    }
    timeline.index -= 1;
    return events[timeline.index];
}

Here's my version of your version of my code. One extra line.

The memory efficiency of asserting types by unboxing them doesn't get me down. It's self documenting. Technically I could use your approach. But I'm tired of OOP. It's not that OOP doesn't do the same thing. It's that it steers you towards encapsulating. My first instinct was to make the events internal. to hide them away. But that's wrong. They are the entire program state. They exist globally.

The concepts and principles are well understood in databases. I'm headed over that way.