asyncGenerator.js

async function* makeFibonacciSequenceGenerator(endIndex = Infinity) {
    let previousNumber = 0;
    let currentNumber = 1;
    
    for (let currentIndex = 0; currentIndex < endIndex; currentIndex++) {
        await new Promise(resolve => setTimeout(resolve, 1000)); // a simple timeout as an example.
        yield currentNumber;
        let nextNumber = currentNumber + previousNumber;
        previousNumber = currentNumber;
        currentNumber = nextNumber;
    }
}

(async () => {
  const fibonacciSequenceGenerator = makeFibonacciSequenceGenerator(6);
  for await (let x of fibonacciSequenceGenerator) {
    console.log(x); // 1, then 1, then 2, then 3, then 5, then 8 (with delay in between).
  }
})();