iterableFibonacciIterator.js

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

  let iterator = {};
  iterator[Symbol.iterator] = () => {
    return {
      next: () => {
        if (currentIndex >= endIndex) { 
            return { value: currentNumber, done: true }; 
        }
        
        const result = { value: currentNumber, done: false };
        const nextNumber = currentNumber + previousNumber;
        previousNumber = currentNumber;
        currentNumber = nextNumber;
        currentIndex++;

        return result;
      }
    }
  };

  return iterator;
}