simpleIterator.js

function makeFibonacciSequenceIterator(endIndex = Infinity) {
  let currentIndex = 0;
  let previousNumber = 0;
  let currentNumber = 1;

  return {
    next: () => {
      if (currentIndex >= endIndex) { 
          return { value: currentNumber, done: true }; 
      }

      let result = { value: currentNumber, done: false };
      let nextNumber = currentNumber + previousNumber;
      previousNumber = currentNumber;
      currentNumber = nextNumber;
      currentIndex++;

      return result;
    }
  };
}