// This method will trigger user permissions
Html5Qrcode.getCameras().then(cameras => {
  /**
   * devices would be an array of objects of type:
   * { id: "id", label: "label" }
   */
  if (devices && devices.length) {
    var cameraId = devices[0].id;
    // .. use this to start scanning.
  }
}).catch(err => {
  // handle err
});

const html5QrCode = new Html5Qrcode(/* element id */ "reader");
html5QrCode.start(
  cameraId, 
  {
    fps: 10,    // Optional frame per seconds for qr code scanning
    qrbox: 250  // Optional if you want bounded box UI
  },
  qrCodeMessage => {
    // do something when code is read
  },
  errorMessage => {
    // parse error, ignore it.
  })
.catch(err => {
  // Start failed, handle it.
});

html5QrCode.stop().then(ignore => {
  // QR Code scanning is stopped.
}).catch(err => {
  // Stop failed, handle it.
});

class Html5Qrcode {
  /**
   * Returns a Promise with list of all cameras supported by the device.
   * 
   * The returned object is a list of result object of type:
   * [{
   *      id: String;     // Id of the camera.
   *      label: String;  // Human readable name of the camera.
   * }]
   */
  static getCameras() // Returns a Promise

  /**
   * Initialize QR Code scanner.
   * 
   * @param {String} elementId - Id of the HTML element. 
   */
  constructor(elementId) {}

  /**
   * Start scanning QR Code for given camera.
   * 
   * @param {String} cameraId Id of the camera to use.
   * @param {Object} config extra configurations to tune QR code scanner.
   *  Supported Fields:
   *      - fps: expected framerate of qr code scanning. example { fps: 2 }
   *          means the scanning would be done every 500 ms.
   *      - qrbox: width of QR scanning box, this should be smaller than
   *          the width and height of the box. This would make the scanner
   *          look like this:
   *          ----------------------
   *          |********************|
   *          |******,,,,,,,,,*****|      <--- shaded region
   *          |******|       |*****|      <--- non shaded region would be
   *          |******|       |*****|          used for QR code scanning.
   *          |******|_______|*****|
   *          |********************|
   *          |********************|
   *          ----------------------
   * @param {Function} qrCodeSuccessCallback callback on QR Code found.
   *  Example:
   *      function(qrCodeMessage) {}
   * @param {Function} qrCodeErrorCallback callback on QR Code parse error.
   *  Example:
   *      function(errorMessage) {}
   * 
   * @returns Promise for starting the scan. The Promise can fail if the user
   * doesn't grant permission or some API is not supported by the browser.
   */
  start(cameraId,
      configuration,
      qrCodeSuccessCallback,
      qrCodeErrorCallback) {}  // Returns a Promise

  /**
   * Stops streaming QR Code video and scanning. 
   * 
   * @returns Promise for safely closing the video stream.
   */
  stop() {} // Returns a Promise
}