GraphQL_Question.js

const SingleRule = new GraphQLObjectType({
  name: 'SingleRule',
  description: 'Single validation rule',
  fields: () => ({
    id: {
      type: GraphQLID,
      description: 'ID of rule'
    },
    fieldName: {
      type: GraphQLString,
      description: 'Field that should be validated'
    },
    validationType: {
      type: RuleType
    },
    rules: new GraphQLList(SingleRule)
  }),
})

const Rules = new GraphQLObjectType({
  name: 'RuleTypeSchema',
  description: 'Validation Rule Type',
  fields: {
    rules: {
      type: new GraphQLList(SingleRule),
      resolve: () => {
        return [
          {
            fieldName: 'firstName',
            validationType: 'all',
            rules: [],
          }
        ]
      }
    },
  }
})

{
  "errors": [
    {
      "message": "The type of SingleRule.rules must be Output Type but got: undefined."
    }
  ]
}