tween.js

function between(subject, options = {}) {
  const {rigidity, alias} = Object.assign({
    rigidity: 10,
    alias: new Map(),
  }, options);

  const keys = Object.keys(subject);
  let last = Object.assign({}, subject);
  let intended = subject;

  function refresh() {
    for (const key of keys) {
      last[key] = last[key] + (intended[key] - last[key]) / rigidity;
      subject[key] = last[key];
    }

    requestAnimationFrame(refresh);
  }

  refresh();

  return function update(values) {
    intended = alias.has(values) ? alias.get(values) : values;
  };
}