1.js

function getMirror(mirrors, offset) {
  Promise.all(
    mirrors.slice(offset * 6, (offset + 1) * 6).map((mirror) => checkMirror(mirror))
  ).then(() => {
    setTimeout(() => getMirror(mirrors, offset + 1), 0)
  })
  .catch(err => console.log(err));
}

fetch(`${basePath}/mirrors.json`)
  .then(response => {
    response.json().then(mirrorsData => {
      getMirror(mirrorsData, 0);
    });
  })
  .catch(err => console.log(err));