var params = {
  a: 1,
  b: 2
};

var data = new FormData();
data.append("json", JSON.stringify(params));

var currentRequest = new Request("/echo/json/", {
  method:"POST",
  body: data
});

var start, complete;
var fetchStart = new Promise(function(_start) {
  start = _start;
})

var fetchComplete = new Promise(function(_complete) {
  complete = _complete;
});
// do stuff when `fetch` is called
fetchStart.then(function(startData) {
  console.log(startData)
});
// do stuff when `fetch` completes
fetchComplete.then(function(completeData) {
  console.log(completeData)
});

var request = fetch(currentRequest);

[request, start({
   "fetchStarted": currentRequest
 })].shift()
.then(function(res) {
  if (res.ok) {
    // resolve `fetchComplete` `Promise` when `fetch` completes
    complete({
       "fetchCompleted": res
     })
  };
  return res.json();
})
.then(function(data) {
  document.body.textContent = JSON.stringify(data)
})
.catch(function(err) {
   // if error, pass error to `fetchComplete`
   complete({
    "fetchCompleted": res,
    "error": err
   });
});

state.showSpinner = true;
Promise.all([fetch(url1), fetch(url2)])
  .then(success => console.log(success))
  .catch(error => console.log(error)
  .finally(() => state.showSpinner = false);