Use new Arrow function syntax instead.

kadira learngraphql defening queries refactor.js

const Post = new GraphQLObjectType({
  name: "Post",
  description: "This represent a Post",
  fields: () => ({
    _id: {type: new GraphQLNonNull(GraphQLString)},
    title: {
      type: new GraphQLNonNull(GraphQLString),
      resolve: post => (post.title || "Does not exist")   
    },
    content: {type: GraphQLString}
  })
});