Custom directive getting dropped during filtering

[AndreaCrotti]

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---

Describe the bug

I'm using this tool to filter queries/mutations based on the existence of a custom directive. The code is simply

const { readFileSync, writeFileSync } = require("fs");
const { buildSchema, printSchema } = require("graphql");
const { mapSchema, getDirective, MapperKind } = require("@graphql-tools/utils");
const config = require("./config.json");
const toFilter = new Set(["Query", "Mutation"]);

function filterSchemaByDirective(schema, directiveName) {
    return mapSchema(schema, {
        [MapperKind.OBJECT_FIELD]: (fieldConfig, _fieldName, typeName, schema) => {
            if (!toFilter.has(typeName)) {
                return fieldConfig;
            }
            const directive = getDirective(schema, fieldConfig, directiveName);
            if (directive) {
                console.log(JSON.stringify(fieldConfig));
                return fieldConfig;
            }
            return null;
        },
    });
}

function filterFile(input, output, directive) {
    const schemaFileContent = readFileSync(input, "utf-8");
    const newSchema = filterSchemaByDirective(buildSchema(schemaFileContent), directive);
    writeFileSync(output, printSchema(newSchema));
}

if (require.main === module) {
    filterFile("transform/hello.graphql", "transform/out.graphql", "expose");
}

And if I feed this in to that code

directive @expose(
  reason: String = "hello"
) on FIELD_DEFINITION | INPUT_FIELD_DEFINITION


input SpectaQLOption { key: String!, value: String! }
directive @spectaql(options: [SpectaQLOption]) on QUERY | MUTATION | FIELD | SCALAR | OBJECT | FIELD_DEFINITION | ARGUMENT_DEFINITION | INTERFACE | UNION | ENUM | ENUM_VALUE | INPUT_OBJECT | INPUT_FIELD_DEFINITION

schema {
  query: Query
}

type Query  {
  ping: Status!
  getList: [String!] @expose @spectaql(options: [{ key: "some", value: "value" }])
  getListError: [String!] @expose @deprecated
  introspection: String
  error: String
  withExtensions: String @spectaql(options: [{ key: "undocumented", value: "true" }])
  iBlowUp: String
}

type Status {
  pong: String!
}

The custom directives expose and spectaql are just dropped:

directive @expose(reason: String = "hello") on FIELD_DEFINITION | INPUT_FIELD_DEFINITION

directive @spectaql(options: [SpectaQLOption]) on QUERY | MUTATION | FIELD | SCALAR | OBJECT | FIELD_DEFINITION | ARGUMENT_DEFINITION | INTERFACE | UNION | ENUM | ENUM_VALUE | INPUT_OBJECT | INPUT_FIELD_DEFINITION

input SpectaQLOption {
  key: String!
  value: String!
}

type Query {
  getList: [String!]
  getListError: [String!] @deprecated
}

type Status {
  pong: String!
}

I can see these directives are still in fieldConfig before I return it so I really don't get why they are dropped.

Expected behavior

I should still see the custom directives.

Environment:

• OS: Mac
• @graphql-tools/utils:^10.0.0"
• NodeJS: 21.7.3