code: function _isBase64(string) {
      const notBase64 = /[^A-Z0-9+\/=]/i;
      const stringLength = string.length;
  
      if (!stringLength || stringLength % 4 !== 0 || notBase64.test(string)) {
        return false;
      }
  
      const firstPaddingChar = string.indexOf('=');
  
      return firstPaddingChar === -1 ||
        firstPaddingChar === stringLength - 1 ||
        (firstPaddingChar === stringLength - 2 && string[stringLength - 1] === '=');
    }
docs: Check if the string a base64 string
  @param string
  @return {boolean}
  @private