throttle.js

/* Returns a wrapped version of `func` which calls `func` at most once every `delay` milliseconds.
** The wrapped function returns a Promise which resolves with the result of calling `func` after the delay has finished.
*/

module.exports = (func, delay) => {
  var lastTimestamp = -Infinity;
  return function (...args) {
    var now = Date.now();
    lastTimestamp = Math.max(now, lastTimestamp + delay);
    return new Promise(resolve => setTimeout(resolve, lastTimestamp - now)).then(() => func.apply(this, args));
  };
};