https://blog.boopathi.in/graphql-optimization-field-filtering/

graphql-optimization-field-filtering.js

const resolvers = {
  Query: {
    async product(_, { id }, __, info) {
      const fields = getFields(info);
      const backendFields = getBackendFields(fields, dependencyMap);
      const backendResponse = await fetch(`/product?id=${id}&fields=${backendFields}`);
      const schemaResponse = getSchemaResponse(backendResponse, fields, transformerMap);
      return schemaResponse;
    }
  }
};

const dependencyMap = {
  name: ["title"],
  price: ["price.currency", "price.amount"],
  stock: ["stock_availability"],
};

const transformerMap = {
  name: resp => resp.title,
  price: resp => `${resp.currency} ${resp.amount}`,
  stock: resp => resp.stock_availability,
};

function getFields(info) {
  return info
    .fieldNodes[0] // TODO: handle all field nodes in other fragments
    .selectionSet
    .selections
    .map(
        selection => // TODO: handle fragments
            selection.name.value
    );
}

function getBackendFields(schemaFields, dependencyMap) {
  // Set helps in deduping
  const backendFields = new Set(
    schemaFields
      .map(field => dependencyMap[field])
      .reduce((acc, field) => [...acc, ...field], [])
  );
  return backendFields.join(",");
}

function getSchemaResponse(
  backendResponse, 
  transformerMap,
  schemaFields
) {
  const schemaResponse = {};
  for (const field of schemaFields) {
    schemaResponse[field] = transformerMap[field](backendResponse);
  }
  return schemaResponse;
}