fib-perf.js

function* fibGenerator() {
  const series = [0, 1];

  while (true) {
    const last = series[1];
    const beforeLast = series.shift();

    const next = last + beforeLast;
    series.push(next);

    yield next;
  }
}

const getFibNumber = n => new Promise(resolve => {
  const iterator = fibGenerator();
  let i = 0;

  (function computeCycle() {
    i += 1;
    const next = iterator.next().value;
    console.info(`Compute progress: ${(i / n) * 100}%`);

    if (i < n) {
      window.requestIdleCallback(computeCycle);
    } else {
      resolve(next);
    }
  })();
});

// Example
getFibNumber(1000000).then(result => console.log(result));