javascript helper to get monotonically increasing values

monotonically-increasing.js

const helpers = {
  *monotonicallyIncreasingGen(start, step) {
    var counter = start;

    while (true) {
      yield counter;
      counter += step;
    }
  },

  monotonicallyIncreasingFunc(start, step) {
    var counter = start - step;

    return function() {
      counter += step;
      return counter;
    };
  }
};

// using ES6 generators
gen = helpers.monotonicallyIncreasingGen(2000, 3000);
console.log(gen.next().value);
console.log(gen.next().value);

// using a closure
next = helpers.monotonicallyIncreasingFunc(2000, 3000);
console.log(next());
console.log(next());

stageful-counter.js

// not happy with this code
const monotonicallyIncreasingCounter = function() {
  var counter = 0,
      step = 1;
  const that = this;;

  return {
    initializeCounter(start, step=1) {
      counter = start - step;
      that.step = step;
    },

    next(step=1) {
      counter += step;
      return counter;
    }
  };
};