Fake typing animation

simulate-typing.js

const trackTime = timing => {
  const now = performance.now();
  if (!timing.startTime) timing.startTime = now;
  const elapsed = now - timing.startTime;
  const {duration} = timing;
  if (duration != null && duration <= elapsed) timing.startTime = null;
  return elapsed;
};

const delay = (callback, duration) => {
  const timing = {duration};
  const tick = () =>
    trackTime(timing) < duration ? requestAnimationFrame(tick) : callback();
  tick();
};

const random = (min, max) =>
  Math.random() * (max - min) + min;

const simulateTyping = ({
  string, target, callback, min = 10, max = 80, iterator = string[Symbol.iterator]()
}) => {
  const {value} = iterator.next();
  if (value)
    delay(() => {
      target.insertAdjacentText("beforeend", value);
      simulateTyping({string, target, callback, min, max, iterator});
    }, random(min, max));
  else
    callback && callback();
  return target;
};

I've implemented it with async/await in https://gist.github.com/rik/56638f4efdd808c9af0e1a43b297d1a2. I find it simpler to understand and tweak.