MrDevil131

Promise.all(
  URL.map(
    (url) => fetch(url)
      .then((response) => `${url} : ${response.status} ${response.statusText}`)
      .catch((e) => `${url} : ${e.message}`),
  ),
)
.then(
  (results) =>
    alert(results.join("\n")),
);

var unresolved = URL.length;
var results = [];
for (i = 0; i < URL.length; i += 1) {
  (function(i) {
    var request = new XMLHttpRequest();
    request.open("GET", URL[i]);
    request.onreadystatechange = function (event) {
      console.log(request);
      if (request.readyState === 4) {
        if (request.status === 200) {
          results[i] = URL[i] + ' РАБОТАЕТ';
        } else {
          results[i] = URL[i] + ' НЕ РАБОТАЕТ';
        }
        unresolved -= 1;
        console.log(unresolved);
        if (unresolved === 0) {
          alert(results.join("\n"));
        }
      }
    }
    request.send();
  })(i);
}