Simplified function of XMLHttpRequest process on Chrome Apps.

main.html

<script src="SimplifiedXMLHttpRequest.js"></script>
<script>
  
  document.getElementById('btn').onclick = function () {
    request({
      url: "http://www.google.com/",
    }, myhandler);
  };
  
  //document.getElementById('btn').onclick = function () {
  //   var requestHeaders = new Array(0);
  //   requestHeaders.push({ key: "Accept", value: "*/*" });
  //   doRequest({
  //     url: "http://hoge/foo.txt",
  //     requestHeaders: requestHeaders,
  //     responseType: "text",
  //     mimeType: "text/plain; charset=shift_jis",
  //   }, myhandler);  
  // }
    
  function myhandler(err, xhr) {
    response = xhr.responseXML || xhr.responseText;
    console.log(response);
    // something to do
  };
</script>

<button id="btn">Exec</button>

manifest.json

{
  ...
  # Chrome Apps aren't so limited by the same origin policy,
  # can talk to remote servers outside of its origin,
  # as long as it first requests cross-origin permissions.
  "permissions": [
    "<all_urls>"
  ]
}

SimplifiedXMLHttpRequest.js

/**
 * pure js simplified XMLHttpRequest function
 *
 * @param {Object} p
 *  {
 *    url:
 *      required.
 *    method:
 *      ('GET' | 'POST')
 *      default is "GET"
 *    formData: eg: `hoge=piyo&moge=fuga`
 *    responseType:
 *      ('document' | 'text')
 *      default is "document"
 *    mimeType:
 *      default is null. 
 *      e.g. "text/plain; charset=shift_jis"
 *    requestHeaders:
 *      array of requestHeader. requestHeader is json likes { key:xxx, value:yyy }.
 *      default is null.
 *    async:
 *      default is true
 *  }
 * @param {Function} - error-first style callback
 * @link https://gist.github.com/akirattii/9084759
 */
var request = function({
  url,
  method = "GET",
  mimeType,
  responseType = "document",
  async = true,
  requestHeaders
}, cb) {
  // logger.debug("Requesting...");
  if (!url) throw Error(`[request] Invalid parameter: url required`);
  let xhr = new XMLHttpRequest();
  if (mimeType) {
    xhr.overrideMimeType(mimeType);
  }
  xhr.responseType = responseType;
  xhr.open(method, url, async);
  if (method && method.toUpperCase() == "POST") {
    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  }
  if (requestHeaders && requestHeaders.length >= 1) {
    for (let len = requestHeaders.length, i = 0; i < len; i++) {
      xhr.setRequestHeader(requestHeaders[i].key, requestHeaders[i].value);
    }
  }
  xhr.onreadystatechange = function(e) {
    if (xhr.readyState == 4) {
      if (xhr.status == 200)
        cb && cb(null, xhr);
      else
        cb && cb(xhr.status, xhr);
    }
  };
  // xhr.onerror = function(err){
  //   console.log("onerror",err);
  // };
  xhr.send(p.formData);
};