async.iterator.js

async function getResponseSize(url) {
  const response = await fetch(url);
  const reader = response.body.getReader();
  let total = 0;

  while (true) {
    const {done, value} = await reader.read();
    if (done) return total;
    total += value.length;
  }
}

async function getResponseSize(url) {
  const response = await fetch(url);
  let total = 0;

  for await (const chunk of response.body) {
    total += chunk.length;
  }
  return total;
}