Fibonacci numbers via generator in JavaScript.

fibonacci-gen.js

function* fibonacci() {
  var earlierPrev = 0;
  var prev = 0;
  var index = -1;
  while (true) {
    index++;
    if (index == 0) {
      yield 0;
    } else if (index == 1) {
      prev = 1;
      yield 1;
    } else {
      let res = earlierPrev + prev;
      earlierPrev = prev;
      prev = res;
      yield res;
    }
  }
}

var f = fibonacci()
f.next()
f.next()
f.next()
f.next()