Schema Directives

Using and implementing custom directives to transform schema types, fields, and arguments

A directive is an identifier preceded by a @ character, optionally followed by a list of named arguments, which can appear after almost any form of syntax in the GraphQL query or schema languages. Here’s an example from the GraphQL draft specification that illustrates several of these possibilities:

directive @deprecated(reason: String = "No longer supported") on FIELD_DEFINITION | ENUM_VALUE

type ExampleType {
  newField: String
  oldField: String @deprecated(reason: "Use `newField`.")
}

As you can see, the usage of @deprecated(reason: ...){:graphql} follows the field that it pertains to (oldField), though the syntax might remind you of “decorators” in other languages, which usually appear on the line above. Directives are typically declared once, using the directive @deprecated ... on ...{:graphql} syntax, and then used zero or more times throughout the schema document, using the @deprecated(reason: ...){:graphql} syntax.

The possible applications of directive syntax are numerous: enforcing access permissions, formatting date strings, auto-generating resolver functions for a particular backend API, marking strings for internationalization, synthesizing globally unique object identifiers, specifying caching behavior, skipping or including or deprecating fields, and just about anything else you can imagine.

This document focuses on directives that appear in GraphQL schemas (as opposed to queries) written in Schema Definition Language, or SDL for short. In the following sections, you will see how custom directives can be implemented and used to modify the structure and behavior of a GraphQL schema in ways that would not be possible using SDL syntax alone.

Using Schema Directives

Most of this document is concerned with implementing schema directives, and some of the examples may seem quite complicated. No matter how many tools and best practices you have at your disposal, it can be difficult to implement a non-trivial schema directive in a reliable, reusable way. Exhaustive testing is essential, and using a typed language like TypeScript is recommended because there are so many different schema types to worry about.

However, the API we provide for using a schema directive is extremely simple. Just import the implementation of the directive, then pass the schema generated by makeExecutableSchema:

import { renameDirective } from 'fake-rename-directive-package'
import { makeExecutableSchema } from '@graphql-tools/schema'

const typeDefs = /* GraphQL */ `
  type Person @rename(to: "Human") {
    name: String!
    currentDateMinusDateOfBirth: Int @rename(to: "age")
  }
`

let schema = makeExecutableSchema({
  typeDefs
})

schema = renameDirective('rename')(schema)

That’s it. The implementation of renameDirective takes care of everything else. If you understand what the directive is supposed to do to your schema, then you do not have to worry about how it works.

For mapping multiple custom schemas you can use a reduce function like so:

import { authDirective } from 'fake-auth-directive-package'
import { lowerDirective } from 'fake-lower-directive-package'
import { renameDirective } from 'fake-rename-directive-package'
import { makeExecutableSchema } from '@graphql-tools/schema'

const typeDefs = /* GraphQL */ `
  type Person @rename(to: "Human") {
    name: String!
    currentDateMinusDateOfBirth: Int @rename(to: "age")
    email: String! @auth(requires: "member") @lower
    phoneNumber: String! @auth(requires: "member")
  }
`

const directiveTransformers = [
  renameDirective('rename').renameDirectiveTransformer,
  authDirective('auth').authDirectiveTransformer,
  lowerDirective('lower').lowerDirectiveTransformer
]

let schema = makeExecutableSchema({ typeDefs })

schema = directiveTransformers.reduce((curSchema, transformer) => transformer(curSchema), schema)

Everything you read below addresses some aspect of how a directive like @rename(to: ...) could be implemented. If that’s not something you care about right now, feel free to skip the rest of this document. When you need it, it will be here.

Implementing Schema Directives

Since the GraphQL specification does not discuss any specific implementation strategy for directives, it’s up to each GraphQL server framework to expose an API for implementing new directives.

GraphQL Tools provides convenient yet powerful tools for implementing directive syntax: the mapSchema and getDirective functions. mapSchema takes two arguments: the original schema, and an object map — pardon the pun — of functions that can be used to transform each GraphQL object within the original schema. mapSchema is a powerful tool, in that it creates a new copy of the original schema, transforms GraphQL objects as specified, and then rewires the entire schema such that all GraphQL objects that refer to other GraphQL objects correctly point to the new set. The getDirective function is straightforward; it extracts any directives (with their arguments) from the SDL originally used to create any GraphQL object.

Here is one possible implementation of the @deprecated directive we saw above:

import { GraphQLSchema } from 'graphql'
import { getDirective, MapperKind, mapSchema } from '@graphql-tools/utils'

function deprecatedDirective(directiveName: string) {
  return {
    deprecatedDirectiveTypeDefs: `directive @${directiveName}(reason: String) on FIELD_DEFINITION | ENUM_VALUE`,
    deprecatedDirectiveTransformer: (schema: GraphQLSchema) =>
      mapSchema(schema, {
        [MapperKind.OBJECT_FIELD](fieldConfig) {
          const deprecatedDirective = getDirective(schema, fieldConfig, directiveName)?.[0]
          if (deprecatedDirective) {
            fieldConfig.deprecationReason = deprecatedDirective['reason']
            return fieldConfig
          }
        },
        [MapperKind.ENUM_VALUE](enumValueConfig) {
          const deprecatedDirective = getDirective(schema, enumValueConfig, directiveName)?.[0]
          if (deprecatedDirective) {
            enumValueConfig.deprecationReason = deprecatedDirective['reason']
            return enumValueConfig
          }
        }
      })
  }
}

To apply this implementation to a schema that contains @deprecated directives, simply pass the necessary typeDefs and schema transformation function to the makeExecutableSchema function in the appropriate positions:

import { deprecatedDirective } from 'fake-deprecated-directive-package'
import { makeExecutableSchema } from '@graphql-tools/schema'

const { deprecatedDirectiveTypeDefs, deprecatedDirectiveTransformer } =
  deprecatedDirective('deprecated')

let schema = makeExecutableSchema({
  typeDefs: [
    deprecatedDirectiveTypeDefs,
    /* GraphQL */ `
      type ExampleType {
        newField: String
        oldField: String @deprecated(reason: "Use \`newField\`.")
      }

      type Query {
        rootField: ExampleType
      }
    `
  ]
})
schema = deprecatedDirectiveTransformer(schema)

We suggest that creators of directive-based schema modification functions allow users to customize the names of the relevant directives, to help users avoid the collision of directive names with existing directives within their schema or other external schema modification functions. Of course, you could hard-code the name of the directive into the function, further simplifying the above examples.

Examples

To appreciate the range of possibilities enabled by mapSchema, let’s examine a variety of practical examples.

Uppercasing Strings

Suppose you want to ensure a string-valued field is converted to uppercase. Though this use case is simple, it’s a good example of a directive implementation that works by wrapping a field’s resolve function:

import { defaultFieldResolver, GraphQLSchema } from 'graphql'
import { makeExecutableSchema } from '@graphql-tools/schema'
import { getDirective, MapperKind, mapSchema } from '@graphql-tools/utils'

function upperDirective(directiveName: string): (schema: GraphQLSchema) => GraphQLSchema {
  return schema =>
    mapSchema(schema, {
      [MapperKind.OBJECT_FIELD]: fieldConfig => {
        const upperDirective = getDirective(schema, fieldConfig, directiveName)?.[0]
        if (upperDirective) {
          const { resolve = defaultFieldResolver } = fieldConfig
          return {
            ...fieldConfig,
            resolve: async function (source, args, context, info) {
              const result = await resolve(source, args, context, info)
              if (typeof result === 'string') {
                return result.toUpperCase()
              }
              return result
            }
          }
        }
      }
    })
}

const upperDirectiveTypeDefs = (directiveName: string) => /* GraphQL */ `
  directive @${directiveName} on FIELD_DEFINITION
`
const applyUpperSchemaTransform = upperDirective('upper')

let schema = makeExecutableSchema({
  typeDefs: [
    upperDirectiveTypeDefs('upper'),
    /* GraphQL */ `
      type Query {
        hello: String @upper
        hello2: String @upperCase
      }
    `
  ],
  resolvers: {
    Query: {
      hello() {
        return 'hello world'
      },
      hello2() {
        return 'hello world'
      }
    }
  }
})

schema = applyUpperSchemaTransform(schema)

Notice how easy it is to handle both @upper and @upperCase with the same upperDirective implementation.

Fetching Data from a REST API

Suppose you’ve defined an object type that corresponds to a REST resource, and you want to avoid implementing resolver functions for every field:

function restDirective(directiveName: string) {
  return {
    restDirectiveTypeDefs: `directive @${directiveName}(url: String) on FIELD_DEFINITION`,
    restDirectiveTransformer: (schema: GraphQLSchema) =>
      mapSchema(schema, {
        [MapperKind.OBJECT_FIELD](fieldConfig) {
          const restDirective = getDirective(schema, fieldConfig, directiveName)?.[0]
          if (restDirective) {
            const { url } = restDirective
            fieldConfig.resolve = () => fetch(url)
            return fieldConfig
          }
        }
      })
  }
}

const { restDirectiveTypeDefs, restDirectiveTransformer } = restDirective('rest')

let schema = makeExecutableSchema({
  typeDefs: [
    restDirectiveTypeDefs,
    /* GraphQL */ `
      type Query {
        people: [Person] @rest(url: "/api/v1/people")
      }
    `
  ]
})

schema = restDirectiveTransformer(schema)

There are many more issues to consider when implementing a real GraphQL wrapper over a REST endpoint (such as how to do caching or pagination), but this example demonstrates the basic structure.

Formatting date strings

Suppose your resolver returns a Date object, but you want to return a formatted string to the client:

function dateDirective(directiveName: string) {
  return {
    dateDirectiveTypeDefs: `directive @${directiveName}(format: String) on FIELD_DEFINITION`,
    dateDirectiveTransformer: (schema: GraphQLSchema) =>
      mapSchema(schema, {
        [MapperKind.OBJECT_FIELD](fieldConfig) {
          const dateDirective = getDirective(schema, fieldConfig, directiveName)?.[0]
          if (dateDirective) {
            const { resolve = defaultFieldResolver } = fieldConfig
            const { format } = dateDirective
            fieldConfig.resolve = async (source, args, context, info) => {
              const date = await resolve(source, args, context, info)
              return formatDate(date, format, true)
            }
            return fieldConfig
          }
        }
      })
  }
}

const { dateDirectiveTypeDefs, dateDirectiveTransformer } = dateDirective('date')

let schema = makeExecutableSchema({
  typeDefs: [
    dateDirectiveTypeDefs,
    /* GraphQL */ `
      scalar Date

      type Query {
        today: Date @date(format: "mmmm d, yyyy")
      }
    `
  ],
  resolvers: {
    Query: {
      today() {
        return new Date(1519688273858).toUTCString()
      }
    }
  }
})
schema = dateDirectiveTransformer(schema)

Of course, it would be even better if the schema author did not have to decide on a specific Date format, but could instead leave that decision to the client. To make this work, the directive just needs to add an additional argument to the field:

import formatDate from 'dateformat'

function formattableDateDirective(directiveName: string) {
  return {
    formattableDateDirectiveTypeDefs: `directive @${directiveName}(
        defaultFormat: String = "mmmm d, yyyy"
      ) on FIELD_DEFINITION
    `,
    formattableDateDirectiveTransformer: (schema: GraphQLSchema) =>
      mapSchema(schema, {
        [MapperKind.OBJECT_FIELD](fieldConfig) {
          const dateDirective = getDirective(schema, fieldConfig, directiveName)?.[0]
          if (dateDirective) {
            const { resolve = defaultFieldResolver } = fieldConfig
            const { defaultFormat } = dateDirective

            if (!fieldConfig.args) {
              throw new Error('Unexpected Error. args should be defined.')
            }

            fieldConfig.args['format'] = {
              type: GraphQLString
            }

            fieldConfig.type = GraphQLString
            fieldConfig.resolve = async (source, { format, ...args }, context, info) => {
              const newFormat = format || defaultFormat
              const date = await resolve(source, args, context, info)
              return formatDate(date, newFormat, true)
            }
            return fieldConfig
          }
        }
      })
  }
}

const { formattableDateDirectiveTypeDefs, formattableDateDirectiveTransformer } =
  formattableDateDirective('date')

let schema = makeExecutableSchema({
  typeDefs: [
    formattableDateDirectiveTypeDefs,
    /* GraphQL */ `
      scalar Date

      type Query {
        today: Date @date
      }
    `
  ],
  resolvers: {
    Query: {
      today() {
        return new Date(1521131357195)
      }
    }
  }
})
schema = formattableDateDirectiveTransformer(schema)

Now the client can specify a desired format argument when requesting the Query.today field, or omit the argument to use the defaultFormat string specified in the schema:

import { graphql } from 'graphql'

graphql(
  schema,
  /* GraphQL */ `
    query {
      today
    }
  `
).then(result => {
  // Logs with the default "mmmm d, yyyy" format:
  console.log(result.data.today)
})

graphql(
  schema,
  /* GraphQL */ `
    query {
      today(format: "d mmm yyyy")
    }
  `
).then(result => {
  // Logs with the requested "d mmm yyyy" format:
  console.log(result.data.today)
})

Enforcing Access Permissions

Imagine a hypothetical @auth directive that takes an argument requires of type Role, which defaults to ADMIN. This @auth directive can appear on an OBJECT like User to set default access permissions for all User fields, as well as appearing on individual fields, to enforce field-specific @auth restrictions:

directive @auth(requires: Role = ADMIN) on OBJECT | FIELD_DEFINITION

enum Role {
  ADMIN
  REVIEWER
  USER
  UNKNOWN
}

type User @auth(requires: USER) {
  name: String
  banned: Boolean @auth(requires: ADMIN)
  canPost: Boolean @auth(requires: REVIEWER)
}

function authDirective(
  directiveName: string,
  getUserFn: (token: string) => { hasRole: (role: string) => boolean }
) {
  const typeDirectiveArgumentMaps: Record<string, any> = {}
  return {
    authDirectiveTypeDefs: `directive @${directiveName}(
      requires: Role = ADMIN,
    ) on OBJECT | FIELD_DEFINITION

    enum Role {
      ADMIN
      REVIEWER
      USER
      UNKNOWN
    }`,
    authDirectiveTransformer: (schema: GraphQLSchema) =>
      mapSchema(schema, {
        [MapperKind.TYPE]: type => {
          const authDirective = getDirective(schema, type, directiveName)?.[0]
          if (authDirective) {
            typeDirectiveArgumentMaps[type.name] = authDirective
          }
          return undefined
        },
        [MapperKind.OBJECT_FIELD]: (fieldConfig, _fieldName, typeName) => {
          const authDirective =
            getDirective(schema, fieldConfig, directiveName)?.[0] ??
            typeDirectiveArgumentMaps[typeName]
          if (authDirective) {
            const { requires } = authDirective
            if (requires) {
              const { resolve = defaultFieldResolver } = fieldConfig
              fieldConfig.resolve = function (source, args, context, info) {
                const user = getUserFn(context.headers.authToken)
                if (!user.hasRole(requires)) {
                  throw new Error('not authorized')
                }
                return resolve(source, args, context, info)
              }
              return fieldConfig
            }
          }
        }
      })
  }
}

function getUser(token: string) {
  const roles = ['UNKNOWN', 'USER', 'REVIEWER', 'ADMIN']
  return {
    hasRole: (role: string) => {
      const tokenIndex = roles.indexOf(token)
      const roleIndex = roles.indexOf(role)
      return roleIndex >= 0 && tokenIndex >= roleIndex
    }
  }
}

const { authDirectiveTypeDefs, authDirectiveTransformer } = authDirective('auth', getUser)

let schema = makeExecutableSchema({
  typeDefs: [
    authDirectiveTypeDefs,
    /* GraphQL */ `
      type User @auth(requires: USER) {
        name: String
        banned: Boolean @auth(requires: ADMIN)
        canPost: Boolean @auth(requires: REVIEWER)
      }

      type Query {
        users: [User]
      }
    `
  ],
  resolvers: {
    Query: {
      users: () => [
        {
          banned: true,
          canPost: false,
          name: 'Ben'
        }
      ]
    }
  }
})
schema = authDirectiveTransformer(schema)

One drawback of this approach is that it does not guarantee fields will be wrapped if they are added to the schema after AuthDirective is applied, and the whole getUser(context.headers.authToken) is a made-up API that would need to be fleshed out. In other words, we’ve glossed over some of the details that would be required for a production-ready implementation of this directive, though we hope the basic structure shown here inspires you to find clever solutions to the remaining problems.

Enforcing Value Restrictions

Suppose you want to enforce a maximum length for a string-valued field:

function lengthDirective(directiveName: string) {
  class LimitedLengthType extends GraphQLScalarType {
    constructor(type: GraphQLScalarType, maxLength: number) {
      super({
        name: `${type.name}WithLengthAtMost${maxLength}`,

        serialize(value: string) {
          const newValue: string = type.serialize(value)
          expect(typeof newValue.length).toBe('number')
          if (newValue.length > maxLength) {
            throw new Error(
              `expected ${newValue.length.toString(10)} to be at most ${maxLength.toString(10)}`
            )
          }
          return newValue
        },

        parseValue(value: string) {
          return type.parseValue(value)
        },

        parseLiteral(ast) {
          return type.parseLiteral(ast, {})
        }
      })
    }
  }

  const limitedLengthTypes: Record<string, Record<number, GraphQLScalarType>> = {}

  function getLimitedLengthType(type: GraphQLScalarType, maxLength: number): GraphQLScalarType {
    const limitedLengthTypesByTypeName = limitedLengthTypes[type.name]
    if (!limitedLengthTypesByTypeName) {
      const newType = new LimitedLengthType(type, maxLength)
      limitedLengthTypes[type.name] = {}
      limitedLengthTypes[type.name][maxLength] = newType
      return newType
    }

    const limitedLengthType = limitedLengthTypesByTypeName[maxLength]
    if (!limitedLengthType) {
      const newType = new LimitedLengthType(type, maxLength)
      limitedLengthTypesByTypeName[maxLength] = newType
      return newType
    }

    return limitedLengthType
  }

  function wrapType<F extends GraphQLFieldConfig<any, any> | GraphQLInputFieldConfig>(
    fieldConfig: F,
    directiveArgumentMap: Record<string, any>
  ): void {
    if (isNonNullType(fieldConfig.type) && isScalarType(fieldConfig.type.ofType)) {
      fieldConfig.type = getLimitedLengthType(fieldConfig.type.ofType, directiveArgumentMap['max'])
    } else if (isScalarType(fieldConfig.type)) {
      fieldConfig.type = getLimitedLengthType(fieldConfig.type, directiveArgumentMap['max'])
    } else {
      throw new Error(`Not a scalar type: ${fieldConfig.type.toString()}`)
    }
  }

  return {
    lengthDirectiveTypeDefs: `directive @${directiveName}(max: Int) on FIELD_DEFINITION | INPUT_FIELD_DEFINITION`,
    lengthDirectiveTransformer: (schema: GraphQLSchema) =>
      mapSchema(schema, {
        [MapperKind.FIELD]: fieldConfig => {
          const lengthDirective = getDirective(schema, fieldConfig, directiveName)?.[0]
          if (lengthDirective) {
            wrapType(fieldConfig, lengthDirective)
            return fieldConfig
          }
        }
      })
  }
}

const { lengthDirectiveTypeDefs, lengthDirectiveTransformer } = lengthDirective('length')

let schema = makeExecutableSchema({
  typeDefs: [
    lengthDirectiveTypeDefs,
    /* GraphQL */ `
      type Query {
        books: [Book]
      }

      type Book {
        title: String @length(max: 10)
      }

      type Mutation {
        createBook(book: BookInput): Book
      }

      input BookInput {
        title: String! @length(max: 10)
      }
    `
  ],
  resolvers: {
    Query: {
      books: () => [{ title: 'abcdefghijklmnopqrstuvwxyz' }]
    },
    Mutation: {
      createBook: (_parent, args) => args.book
    }
  }
})
schema = lengthDirectiveTransformer(schema)

Note that new types can be added to the schema with ease, but that each type must be uniquely named.

Synthesizing Unique IDs

Suppose your database uses incrementing IDs for each resource type, so IDs are not unique across all resource types. Here’s how you might synthesize a field called uid that combines the object type with various field values to produce an ID that’s unique across your schema:

import { createHash } from 'crypto'
import { GraphQLID } from 'graphql'

function uniqueIDDirective(directiveName: string) {
  return {
    uniqueIDDirectiveTypeDefs: `directive @${directiveName}(name: String, from: [String]) on OBJECT`,
    uniqueIDDirectiveTransformer: (schema: GraphQLSchema) =>
      mapSchema(schema, {
        [MapperKind.OBJECT_TYPE]: type => {
          const uniqueIDDirective = getDirective(schema, type, directiveName)?.[0]
          if (uniqueIDDirective) {
            const { name, from } = uniqueIDDirective
            const config = type.toConfig()
            config.fields[name] = {
              type: GraphQLID,
              description: 'Unique ID',
              args: {},
              resolve(object: any) {
                const hash = createHash('sha1')
                hash.update(type.name)
                for (const fieldName of from) {
                  hash.update(String(object[fieldName]))
                }
                return hash.digest('hex')
              }
            }
            return new GraphQLObjectType(config)
          }
        }
      })
  }
}

const { uniqueIDDirectiveTypeDefs, uniqueIDDirectiveTransformer } = uniqueIDDirective('uniqueID')

let schema = makeExecutableSchema({
  typeDefs: [
    uniqueIDDirectiveTypeDefs,
    /* GraphQL */ `
      type Query {
        people: [Person]
        locations: [Location]
      }

      type Person @uniqueID(name: "uid", from: ["personID"]) {
        personID: Int
        name: String
      }

      type Location @uniqueID(name: "uid", from: ["locationID"]) {
        locationID: Int
        address: String
      }
    `
  ],
  resolvers: {
    Query: {
      people: () => [
        {
          personID: 1,
          name: 'Ben'
        }
      ],
      locations: () => [
        {
          locationID: 1,
          address: '140 10th St'
        }
      ]
    }
  }
})
schema = uniqueIDDirectiveTransformer(schema)

GraphQL modules usage

If you create your application with GraphQL modules please use the schemaBuilder parameter in createApplication() to apply the directive implementation:

import { anyDirective } from 'any-directive-package'
import { makeExecutableSchema } from '@graphql-tools/schema'

const application = createApplication({
    ...
    schemaBuilder: ({ typeDefs, resolvers }) => {
        // Build the schema from modules
        let schema = makeExecutableSchema({
            typeDefs,
            resolvers: resolvers
        })

        // Apply the directive transformer, i.e. rename directive
        const patchedSchema = anyDirective('rename')(schema);

        return patchedSchema;
    }
})

Declaring Schema Directives

SDL syntax requires declaring the names, argument types, default argument values, and permissible locations of any available directives. We have shown one approach above to doing so. If you’re implementing a reusable directive for public consumption, you will probably want to either guide your users as to how properly declare their directives, or export the required SDL syntax as above so that users can pass it to makeExecutableSchema. These techniques can be used in combination, i.e. you may wish to export the directive syntax and provide instructions on how to structure any dependent types. Take a second look at the auth example above to see how this may be done and note the interplay between the directive definition and the Role type.

What about Query Directives?

The directive syntax can also appear in GraphQL queries sent from the client. Query directive implementation can be performed within GraphQL resolver using similar techniques as the above. In general, however, schema authors should consider using field arguments wherever possible instead of query directives, with query directives most useful for annotating the query with metadata affecting the execution algorithm itself, e.g. defer, stream, etc.

In theory, access to the query directives is available within the info resolver argument by iterating through each fieldNode of info.fieldNodes, although, as above use of query directives within standard resolvers is not necessarily recommended.

What about `directiveResolvers`?

The makeExecutableSchema function is used to take a directiveResolvers option that could be used for implementing certain kinds of @directives on fields that have resolver functions.

The new abstraction is more general since it can visit any kind of schema syntax, and do much more than just wrap resolver functions. The old directiveResolvers API can be implemented with the above new API as follows:

export function attachDirectiveResolvers(
  schema: GraphQLSchema,
  directiveResolvers: IDirectiveResolvers
): GraphQLSchema {
  // ... argument validation ...

  return mapSchema(schema, {
    [MapperKind.OBJECT_FIELD]: fieldConfig => {
      const newFieldConfig = { ...fieldConfig }

      const directives = getDirectives(schema, fieldConfig)
      for (const directive of directives) {
        const directiveName = directive.name
        if (directiveResolvers[directiveName]) {
          const resolver = directiveResolvers[directiveName]
          const originalResolver =
            newFieldConfig.resolve != null ? newFieldConfig.resolve : defaultFieldResolver
          const directiveArgs = directive.args
          newFieldConfig.resolve = (source, originalArgs, context, info) => {
            return resolver(
              () =>
                new Promise((resolve, reject) => {
                  const result = originalResolver(source, originalArgs, context, info)
                  if (result instanceof Error) {
                    reject(result)
                  }
                  resolve(result)
                }),
              source,
              directiveArgs,
              context,
              info
            )
          }
        }
      }

      return newFieldConfig
    }
  })
}

What about Code-First Schemas?

You can use schema transformation functions with code-first schemas as well. By default, if a directives key exists within the extensions field for a given GraphQL entity, the getDirectives function will retrieve the directive data from the GraphQL entity’s extensions.directives data rather than from the SDL. This, of course, allows schemas created without SDL to use any schema transformation functions created for directive use, as long as they define the necessary data within the GraphQL entity extensions.

This behavior can be customized! The getDirectives function takes a third argument, pathToDirectivesInExtensions, an array of strings, that allows customization of this path to directive data within extensions, which is set to ['directives'] by default. We recommend allowing end users to customize this path similar to how the directive name can be customized above.

See this graphql-js issue for more information on directives with code-first schemas. We follow the Gatsby and graphql-compose convention of reading directives from the extensions field, but allow customization as above.

Full `mapSchema` API

How can you customize schema mapping? The second argument provided to mapSchema is an object of type SchemaMapper that can specify individual mapping functions.

GraphQL’s objects are mapped according to the following algorithm:

Types are mapped. The most general matching mapping function available will be used, i.e. inclusion of a MapperKind.TYPE will cause all types to be mapped with the specified mapper. Specifying MapperKind.ABSTRACT_TYPE and MapperKind.MAPPER.QUERY mappers will cause the first mapper to be used for interfaces and unions, the latter to be used for the root query object type, and all other types to be ignored.
Enum values are mapped. If all you want to do to an enum is to change one value, it is more convenient to use a MapperKind.ENUM_VALUE mapper than to iterate through all values on your own and recreate the type – although that would work!
Fields are mapped. Similar to above, if you want to modify a single field, mapSchema can do the iteration for you. You can subspecify MapperKind.OBJECT_FIELD or MapperKind.ROOT_FIELD to select a limited subset of fields to map.
Arguments are mapped. Similar to above, you can subspecify MapperKind.ARGUMENT if you want to modify only an argument. mapSchema can iterate through the types and fields for you.
Directives are mapped if MapperKind.DIRECTIVE is specified.

export interface SchemaMapper {
  [MapperKind.TYPE]?: NamedTypeMapper
  [MapperKind.SCALAR_TYPE]?: ScalarTypeMapper
  [MapperKind.ENUM_TYPE]?: EnumTypeMapper
  [MapperKind.COMPOSITE_TYPE]?: CompositeTypeMapper
  [MapperKind.OBJECT_TYPE]?: ObjectTypeMapper
  [MapperKind.INPUT_OBJECT_TYPE]?: InputObjectTypeMapper
  [MapperKind.ABSTRACT_TYPE]?: AbstractTypeMapper
  [MapperKind.UNION_TYPE]?: UnionTypeMapper
  [MapperKind.INTERFACE_TYPE]?: InterfaceTypeMapper
  [MapperKind.ROOT_OBJECT]?: ObjectTypeMapper
  [MapperKind.QUERY]?: ObjectTypeMapper
  [MapperKind.MUTATION]?: ObjectTypeMapper
  [MapperKind.SUBSCRIPTION]?: ObjectTypeMapper
  [MapperKind.ENUM_VALUE]?: EnumValueMapper
  [MapperKind.FIELD]?: GenericFieldMapper<GraphQLFieldConfig<any, any> | GraphQLInputFieldConfig>
  [MapperKind.OBJECT_FIELD]?: FieldMapper
  [MapperKind.ROOT_FIELD]?: FieldMapper
  [MapperKind.QUERY_ROOT_FIELD]?: FieldMapper
  [MapperKind.MUTATION_ROOT_FIELD]?: FieldMapper
  [MapperKind.SUBSCRIPTION_ROOT_FIELD]?: FieldMapper
  [MapperKind.INTERFACE_FIELD]?: FieldMapper
  [MapperKind.COMPOSITE_FIELD]?: FieldMapper
  [MapperKind.INPUT_OBJECT_FIELD]?: InputFieldMapper
  [MapperKind.ARGUMENT]?: ArgumentMapper
  [MapperKind.DIRECTIVE]?: DirectiveMapper
}

export type NamedTypeMapper = (
  type: GraphQLNamedType,
  schema: GraphQLSchema
) => GraphQLNamedType | null | undefined

export type ScalarTypeMapper = (
  type: GraphQLScalarType,
  schema: GraphQLSchema
) => GraphQLScalarType | null | undefined

export type EnumTypeMapper = (
  type: GraphQLEnumType,
  schema: GraphQLSchema
) => GraphQLEnumType | null | undefined

export type EnumValueMapper = (
  value: GraphQLEnumValueConfig,
  typeName: string,
  schema: GraphQLSchema
) => GraphQLEnumValueConfig | [string, GraphQLEnumValueConfig] | null | undefined

export type CompositeTypeMapper = (
  type: GraphQLObjectType | GraphQLInterfaceType | GraphQLUnionType,
  schema: GraphQLSchema
) => GraphQLObjectType | GraphQLInterfaceType | GraphQLUnionType | null | undefined

export type ObjectTypeMapper = (
  type: GraphQLObjectType,
  schema: GraphQLSchema
) => GraphQLObjectType | null | undefined

export type InputObjectTypeMapper = (
  type: GraphQLInputObjectType,
  schema: GraphQLSchema
) => GraphQLInputObjectType | null | undefined

export type AbstractTypeMapper = (
  type: GraphQLInterfaceType | GraphQLUnionType,
  schema: GraphQLSchema
) => GraphQLInterfaceType | GraphQLUnionType | null | undefined

export type UnionTypeMapper = (
  type: GraphQLUnionType,
  schema: GraphQLSchema
) => GraphQLUnionType | null | undefined

export type InterfaceTypeMapper = (
  type: GraphQLInterfaceType,
  schema: GraphQLSchema
) => GraphQLInterfaceType | null | undefined

export type DirectiveMapper = (
  directive: GraphQLDirective,
  schema: GraphQLSchema
) => GraphQLDirective | null | undefined

export type GenericFieldMapper<F extends GraphQLFieldConfig<any, any> | GraphQLInputFieldConfig> = (
  fieldConfig: F,
  fieldName: string,
  typeName: string,
  schema: GraphQLSchema
) => F | [string, F] | null | undefined

export type FieldMapper = GenericFieldMapper<GraphQLFieldConfig<any, any>>

export type ArgumentMapper = (
  argumentConfig: GraphQLArgumentConfig,
  fieldName: string,
  typeName: string,
  schema: GraphQLSchema
) => GraphQLArgumentConfig | [string, GraphQLArgumentConfig] | null | undefined

export type InputFieldMapper = GenericFieldMapper<GraphQLInputFieldConfig>