code: function assertFormat (key, value, descriptor) {
    var fn = JWT.formats[descriptor.format]
  
    if (!fn) {
      throw new Error(
        descriptor.format + ' is not a recognized format'
      )
    }
  
    if (!fn(value)) {
      throw new Error(
        key + ' must conform to ' + descriptor.format + ' format'
      )
    }
  
    return true
  }
docs: Assert Format