test.js

var requestCallback;

chrome.storage.sync.get({
  redirect_domain : "zerx.co",
  domain_checking_interval : 5000
}, function(items) {
  console.log(items.domain_checking_interval);

  setInterval(function() {
    console.log('Domain name updated.');
    check_domain();

    if (typeof requestCallback === "function") {
        chrome.webRequest.onBeforeRequest.removeListener(requestCallback);
        requestCallback = null;
    }
    
    chrome.webRequest.onBeforeRequest.addListener(
      function(details) {
        //requestCallback = this;
        redirected_url = "http://" + items.redirect_domain;
        console.log("Redirected to: " + redirected_url);
        return {redirectUrl: redirected_url};
      },
      {
        urls: ["http://zerx.co/*"],
        types: ["main_frame"]
      },
      ["blocking"]
    );

  }, items.domain_checking_interval);
});