ニコニコ動画でgetFlvを呼び出す関数

nicovideo_getFlv.js

(function() {
  if (!('nicovideo_getFlv' in window)) {
    const Getter = function(videoID) {
      this._videoID = videoID;
      this._callbacks = [];
    };

    Getter.prototype = {
      _videoID: null,
      _lock: false,
      _callbacks: null,
      _cacheStr: null,
      _cacheObj: null,
      get: function(callback) {
        if (typeof callback === 'function')
          this._callbacks.push(callback);
        if (this._cacheStr !== null) {
          this._callCallback();
          return;
        }

        if (this._lock) return;
        this._lock = true;
        let self = this;
        this._updateCache(function() {
          self._callCallback();
          self._lock = false;
        });
      },
      _callCallback: function() {
        let original = this._cacheObj, str = this._cacheStr;
        this._callbacks.forEach(function(c) {
          try {
            let obj = {};
            for (let key in original) {
              if (original.hasOwnProperty(key))
                obj[key] = original[key];
            }
            c(obj, str);
          } catch(e) {
            GM_log(e);
          }
        });
        this._callbacks = [];
      },
      _updateCache: function(callback) {
        let self = this;
        GM_xmlhttpRequest( {
          method: 'GET',
          url: 'http://flapi.nicovideo.jp/api/getflv/' + this._videoID,
          headers: {
            'User-Agent': 'Mozilla/5.0 Greasemonkey; NicoGetFlv'
          },
          onload: function(response) {
            self._cacheStr = response.responseText;
            self._cacheObj = self._parse(response.responseText);
            callback();
          },
          onerror: function(response) {
            self._cacheStr = null;
            self._cacheObj = null;
            callback();
          }
        });
      },
      _parse: function(str) {
        let result = {};
        let pairs = str.split('&').map(
          function(pair) pair.split('=').map(decodeURIComponent)
        ).forEach(function([key, value]) {
          switch (key) {
          case 'rpu':
            result[key] = JSON.parse(value);
            break;
          case 'ng_up':
            result[key] = value.split('&').map(
              function(data) {
                let [before, after] = data.split('=').map(decodeURIComponent);
                let replaceAll = false;
                if (before[0] === '*') {
                  before = before.slice(1);
                  replaceAll = true;
                }
                return [ before, after, replaceAll ];
              }
            );
            break;
          default:
            result[key] = value;
            break;
          }
        });
        return result;
      }
    };

    let getters = {};
    getters.__proto__ = null;
    window.nicovideo_getFlv = function(videoID, callback) {
      let getter = null;
      if (videoID in getters) {
        getter = getters[videoID];
      } else {
        getter = new Getter(videoID);
        getters[videoID] = getter;
      }
      getter.get(callback);
    };
  };
})();

sample.js

nicovideo_getFlv('sm9', function(obj, responseText) {
  // http://flapi.nicovideo.jp/api/getflv/sm9 の内容を取得する
  alert(responseText); // 上記URLにアクセスして得られる文字列
  alert(uneval(obj));  // 得られた文字列をパースしたもの
}