Helper: Check target URL using XMLHttpRequest before we execute a callback accordingly. No Ajax/Promise needed.

ifTargetPageExists.js

/**
 * Ensure that target page exists before we actually do something.
 *
 * @param {string} targetUrl - Absolute target URL to be tested
 * @param {function} [successCallback] - Function to be executed on success
 * @param {function} [failCallback] - Function to be executed on fail
 */
function checkTargetFirst(targetUrl, successCallback, failCallback) {
  var reader = new XMLHttpRequest();

  successCallback = successCallback || function () {};
  failCallback = failCallback || function () {};

  //// For debugging: You override targetUrl here and play arround
  //// by commenting out the success and fail callback.
  // targetUrl = "http://localhost:4502/content/client-site/ch_CH.html";

  reader.open('get', targetUrl, true);
  reader.onreadystatechange = function () {
    if (reader.readyState === 4) {

      // Check whether request for the file failed or succeeded
      if ((reader.status === 200) || (reader.status === 0)) {
        successCallback();
      } else {
        failCallback();
      }
    }
  };

  reader.send(null);
}