Windowed fetch with slots

windowedFetch.js

const delay = (ms) => new Promise((res) => setTimeout(res, ms));

const createWindowedFetch = (windowSize = 1000, reqPerWindow = 30) => {
  const slots = [0];
  const start = new Date().getTime();
  let lastSlot = start;

  return async (url, requestInit) => {
    let firstFreeSlotIndex = slots.findIndex(
      (allocated) => allocated < reqPerWindow
    );
    if (firstFreeSlotIndex === -1) {
      firstFreeSlotIndex = slots.length;
    }
    slots[firstFreeSlotIndex] = (slots[firstFreeSlotIndex] ?? 0) + 1;
    const currentSlotStart =
      start +
      Math.floor((new Date().getTime() - start) / windowSize) * windowSize;
    const matchedSlotStart = currentSlotStart + firstFreeSlotIndex * windowSize;

    if (firstFreeSlotIndex) {
      await delay(matchedSlotStart - new Date().getTime());
    }

    if (lastSlot !== matchedSlotStart) {
      slots.shift();
      lastSlot = matchedSlotStart;
    }

    const response = await fetch(url, requestInit);
    if (!response.ok)
      throw new Error(`Request failed: ${await response.text()}`);
    return response.json();
  };
};

const windowedFetch = createWindowedFetch();

async function test() {
  for (let i = 0; i < 100; i++) {
    const start = performance.now();
    windowedFetch("http://127.0.0.1:3000/test.json").then(() =>
      console.log(`Request ${i} took ${performance.now() - start} ms`)
    );
  }
}

test();