nested-fetch.js

async function getAllMyData() {
   const rootResponse = await fetch('https://www.myapi.com/foo', { ... });

   const rootData = await rootResponse.json();

  // this gets all the ids we need for the nested requests from the data
  // in the first request
  const ids = rootData.bars.map(bar => bar.id);

  // fetch all the details of all the bar objects. This makes multiple
  // requests that run in parallel and waits for all of them. The barData
  // value will be an array of objects, one for each request.
  const allBarData = await Promise.all(ids.map(async (id) => {
    const response = await fetch('https://www.myapi.com/bar/' + id);
    return await response.json();
  }));

  // you could build a new returned object here that combines the
  // allBarData with the rootData
  return allBarData;
}