iterator.js

/*
  Simple iterator implementation that allows to calculate sequenced values.
  Next value is calculated by the `update` function.
  
  This construction might come in handy when it's required to generate a sequence,
  but there's also no need to keep the history of previous values.
  
  Note: could also be elegangtly descibed via class syntax with private fields.
  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields
*/
function Iterator(update = x => x, initialValue = 0) {
  this.value = initialValue;
  this.next = () => this.value = update(this.value);
}