text: // Copyright 2018 The AMPHTML Authors
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //      http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  class NavigationWarning {
    constructor(win) {
      this.win = win;
    }
  
    enable() {
      this.win.onbeforeunload = () => true;
    }
  
    disable() {
      this.win.onbeforeunload = null;
    }
  }
  
  export default new NavigationWarning(window);
  