cross-domain post form using iframe

crossdomain-post-form.js

let url = "http://example.host/"; // target to post form

let params = [
  { name: "a", value: encodeURIComponent("1") },
  { name: "b", value: encodeURIComponent("2") },
];

crossDomainPost(url, params);

function crossDomainPost(url, params) {
  // Add the iframe with a unique name
  let iframe = document.createElement("iframe");
  let uniqueString = "CHANGE_THIS_TO_SOME_UNIQUE_STRING";
  document.body.appendChild(iframe);
  iframe.style.display = "none";
  iframe.contentWindow.name = uniqueString;

  // construct a form with hidden inputs, targeting the iframe
  let form = document.createElement("form");
  form.target = uniqueString;
  form.action = url;
  form.method = "POST";

  addParams2Form(params, form);

  document.body.appendChild(form);
  form.submit();

  function addParams2Form(arr, form) {
    arr.forEach(function(item) {
      let input = document.createElement("input");
      input.type = "hidden";
      input.name = item.name;
      input.value = item.value;
      form.appendChild(input);
    });
  }
}