Iterator

iterator.js

range = (start, end, step = 1) => {
  return { 
    *[Symbol.iterator]() {
      let by = start  >= end && step > 0 ? step*(-1) : step;
      let current = start;
      while ((start < end && current < end) || (start > end && current > end)) {
        yield current;
        current = current + by;
      }
      return end;
    },
    map(fn) {
      let result = [];
      for (let d of range(start, end)) {
        result.push(fn(d));
      }
      return result;
    },
    reduce(fn) {
      let init = start;
      for(let d of range(start, end)) {
        init = fn(init, d);
      }
      return init;
    }
  }
}

for (i of range(100, 0, 10)) console.log(i); // 100 90 80 70 60 50 40 30 20 10

const powerOfTwo = (x) => Math.pow(2, x)
range(0, 8).map(powerOfTwo) // [1, 2, 4, 8, 16, 32, 64, 128]