Inject external JavaScript like a ninja

ninjectscript.js

var ninjectScript = (function(window, document) {
  var head = document.head || document.getElementsByTagName('head')[0],
    BlobBuilder,
    builderPrefixes = [
      'webkit', 'Webkit', 'WebKit',
      'moz', 'Moz', 'o', 'O',
      'ms', 'Ms', 'MS',
      'khtml', 'Khtml', ''
    ];

  return function(src, loadCallback) {
    var request = getXHR();
    if(!request) return injectScriptOld(src, loadCallback);
    request.responseType = 'blob';
    request._load = loadCallback;
    request.onload = onXHRLoad;
    request.open('GET', src);
    request.send();
  };

  function injectScriptOld(src, loadCallback) {
    var script = document.createElement('script');
    script._load = loadCallback;
    script.onload = onScriptLoad;
    script.src = src;
    head.appendChild(script);
  }
  
  function onXHRLoad() {
    var response = this.response, useText;
    if(typeof response === 'string') {
      var blobResponse = getBlob(response, 'text/javascript');
      useText = !blobResponse;
      response = blobResponse || response;
    }
    var script = document.createElement('script');
    if(useText) {
      if(Object.getOwnPropertyDescriptor(script, 'textContent'))
        script.textContent = response;
      else
        script.innerHTML = response;
      setTimeout(onScriptContentLoad, 1, script, this._load);
    } else {
      script._load = this._load;
      script.onload = onScriptLoad;
      script.async = true;
      script.src = URL.createObjectURL(response);
    }
    head.appendChild(script);
  }
  
  function onScriptLoad(e) {
    try {
      URL.revokeObjectURL(this.src);
    } catch(ex) {}
    onScriptContentLoad(this, this._load);
  }

  function onScriptContentLoad(script, loadCallback) {
    try {
      script.parentElement.removeChild(script);
    } catch(err) {}
    try {
      if(typeof loadCallback === 'function')
        loadCallback.call(script);
    } catch(err) {}
  }

  function getXHR() { 
    if(window.XMLHttpRequest) return new window.XMLHttpRequest();
    try {
      return new ActiveXObject('MSXML2.XMLHTTP.6.0');
    } catch(err) {}
    try {
      return new ActiveXObject('MSXML2.XMLHTTP.3.0');
    } catch(err) {} 
  }

  function getBlob(content, type) {
    if(window.Blob)
      return new window.Blob([content], { type: type });
    if(!BlobBuilder) {
      if(BlobBuilder === null) return;
      if(window.BlobBuilder)
        BlobBuilder = window.BlobBuilder;
      else
        for(var i = 0; !BlobBuilder || i < builderPrefixes.length; i++)
          BlobBuilder = window[builderPrefixes[i] + 'BlobBuilder'];
      if(!BlobBuilder) BlobBuilder = null;
    }
    var builder = new BlobBuilder();
    builder.append(content);
    return builder.getBlob(type);
  }
})(window, document);

// Optional snippet that can be placed at the end of your script
// It will make your inline <script> tag become invisible:
(function(element) {
  try { if(element) element.parentElement.removeChild(element); } catch(err) {}
})(document.currentScript);

// Usage:
ninjectScript('https://example.com/something/invisible.js', function() {
  console.log('Script loaded with Ninjector, and you can\'t see where it has loadad to, even inspecting it:\n%s', this.src);
});