eslint-plugin-nestjs-graphql

This plugin intends to prevent issues with returning the wrong type from NestJS GraphQL resolvers. Relevant to Code first approach.

Rules

The plugin supports rules:

matching-return-type matching-resolve-field-parent-type matching-args-type matching-field-type require-resolve-field-for-nested-models require-resolver-type-arg-with-resolve-field no-optional-fields-in-object-type no-redundant-field-decorator

Motivation

matching-return-type

When Code first approach is used, NestJS generates schema based on the decorators such as ResolveField, Query, or Mutation which define the type of the returned value. However, the type of the returned value is not checked by TypeScript compiler.

A query defined as:

  @Query(returns => Author)
  async author(@Args('id', { type: () => Int }) id: number) {
    return this.authorsService.findOneById(id);
  }

can be implemented to return any type of value, e.g. Promise<string>. This will not be caught by TypeScript compiler, but will result in runtime error when the GraphQL schema is generated.

This rule aims to solve this issue by checking the type of the returned value.

Valid

  @Query(returns => Author)
  async author(@Args('id', { type: () => Int }) id: number): Author {
    return this.authorsService.findOneById(id);
  }

  @Query(returns => Author)
  async author(@Args('id', { type: () => Int }) id: number): Promise<Author> {
    return this.authorsService.findOneById(id);
  }

  @Query(returns => [Author])
  async author(@Args('id', { type: () => Int }) id: number): Promise<Author[]> {
    return this.authorsService.findOneById(id);
  }

  @Query(returns => [Author], { nullable: true })
  async author(@Args('id', { type: () => Int }) id: number): Promise<Author[] | null> {
    return this.authorsService.findOneById(id);
  }

Invalid

  @Query(returns => Author)
  async author(@Args('id', { type: () => Int }) id: number): string {
    return this.authorsService.findOneById(id);
  }

  @Query(returns => Author)
  async author(@Args('id', { type: () => Int }) id: number): Promise<Author | null> {
    return this.authorsService.findOneById(id);
  }

  @Query(returns => Author)
  async author(@Args('id', { type: () => Int }) id: number): Promise<Author[]> {
    return this.authorsService.findOneById(id);
  }

matching-resolve-field-parent-type

When resolving a field, the @Parent() decorator's type can mismatch the type returned from the @Resolver() decorator of the class. This may result in runtime error or unexpected behavior.

This rule aims to solve this issue by checking the type of the @Parent against @Resolver().

Valid

  @Resolver(() => Author)
  class AuthorResolver {
    @ResolveField(() => [Book])
    async books(@Parent() author: Author): Promise<Book[]> {
      return this.booksService.findAllByAuthorId(author.id);
    }
  }

  @Resolver(Author)
  class AuthorResolver {
    @ResolveField(returns => [Book])
    async books(@Parent() author: Author): Promise<Book[]> {
      return this.booksService.findAllByAuthorId(author.id);
    }
  }

Invalid

  @Resolver()
  class AuthorResolver {
    @ResolveField(returns => [Book])
    async books(@Parent() author: Author): Promise<Book[]> {
      return this.booksService.findAllByAuthorId(author.id);
    }
  }

  @Resolver(Author)
  class AuthorResolver {
    @ResolveField(returns => [Book])
    async books(@Parent() author: Book): Promise<Book[]> {
      return this.booksService.findAllByAuthorId(author.id);
    }
  }

require-resolve-field-for-nested-models

GraphQL object relationships should be resolved in dedicated resolvers using @ResolveField. Declaring nested models directly on another @ObjectType() can lead to unexpected schema nesting and makes it harder to reuse resolvers.

Valid

@ObjectType()
class User {
  @Field(() => ID)
  id!: string;
}

@Resolver(() => User)
class UserResolver {
  @ResolveField(() => Profile)
  profile(@Parent() user: User): Promise<Profile> {
    return this.profileService.byUserId(user.id);
  }
}

Invalid

@ObjectType()
class User {
  @Field(() => Profile)
  profile!: Profile;
}

matching-args-type

Mirrors matching-return-type, but on @Args parameters. When @Args('name', { type: () => X, nullable?: ... }) is given, this rule verifies that the TypeScript parameter type matches the declared type, and that the nullable option agrees with the annotation. Array nullability is modelled distinctly:

| nullable option | expected TS shape | | ----------------------- | -------------------------- | | true | X \| null or X[] \| null | | 'items' | (X \| null)[] | | 'itemsAndList' | (X \| null)[] \| null |

All three of | null, | undefined, and ?: are accepted to express nullability at the list level, but | null is recommended for consistency with no-optional-fields-in-object-type. When @Args has no options object, analysis is skipped — the compiler plugin infers from TypeScript, so there's nothing to drift.

Valid

  @Query(() => User)
  user(
    @Args('id', { type: () => Int }) id: number,
    @Args('filter', { type: () => String, nullable: true }) filter: string | null,
    @Args('tags', { type: () => [String], nullable: 'items' }) tags: (string | null)[],
  ) { ... }

Invalid

  @Query(() => User)
  user(@Args('id', { type: () => Int }) id: string) { ... }  // Int vs string

  @Query(() => User)
  user(@Args('id', { type: () => Int, nullable: true }) id: number) { ... }  // nullable: true without | null

  @Query(() => [User])
  users(
    @Args('ids', { type: () => [Int], nullable: 'items' }) ids: number[],  // decorator says items nullable, type doesn't
  ) { ... }

require-resolver-type-arg-with-resolve-field

@Resolver() without a parent type argument is incompatible with @ResolveField methods (they need a parent to attach to). This rule fails the class-level @Resolver() decorator whenever it contains any @ResolveField. Complements matching-resolve-field-parent-type, which only fires when a @Parent() parameter is present.

Valid

@Resolver(() => User)
class UserResolver {
  @ResolveField(() => [Post])
  posts(@Parent() user: User) { ... }
}

Invalid

@Resolver()
class UserResolver {
  @ResolveField(() => [Post])
  posts(@Parent() user: User) { ... }
}

matching-field-type

Parity rule for @Field-decorated properties on @ObjectType, @InputType, and @ArgsType classes. Same contract as matching-args-type — verifies the TypeScript property type matches the decorator's type argument, and that nullable agrees with the annotation (including the 'items' and 'itemsAndList' forms for arrays). Properties without an explicit @Field are skipped: the NestJS GraphQL compiler plugin infers their type from TypeScript.

Valid

@ObjectType()
class User {
  @Field(() => ID) id!: string;
  @Field(() => String, { nullable: true }) nickname!: string | null;
  @Field(() => [String], { nullable: 'items' }) tags!: (string | null)[];
}

Invalid

@ObjectType()
class User {
  @Field(() => Int) name!: string;                // Int vs string
  @Field(() => String) nickname!: string | null;  // TS nullable, decorator isn't
}

no-redundant-field-decorator

Autofixes away @Field decorators that carry no information the NestJS GraphQL compiler plugin can't derive from TypeScript — i.e. no type: () => X function and a TS type that's one of string, number, boolean. Trivial options (nullable, description) don't rescue the decorator; any other option does. This rule assumes the compiler plugin is enabled in your nest-cli.json — otherwise @Field is mandatory and should not be stripped.

Valid (decorator is load-bearing — skipped)

@ObjectType()
class User {
  @Field(() => Int) count!: number;                       // explicit type function
  @Field({ complexity: 5 }) name!: string;                // non-trivial option
  @Field() profile!: Profile;                             // non-primitive TS type
}

Invalid (autofixable — decorator will be removed)

@ObjectType()
class User {
  @Field() name!: string;
  @Field({ nullable: true }) nickname!: string | null;
  @Field({ description: 'The user display name' }) displayName!: string;
}

no-optional-fields-in-object-type

Optional (?) properties on @ObjectType classes are easy to forget to populate — the value is silently undefined and the field is absent from the response. Requiring | null instead forces an explicit assignment at every construction site, so a missing value becomes a TypeScript error.

Valid

@ObjectType()
class User {
  id!: string;
  nickname!: string | null;
}

Invalid

@ObjectType()
class User {
  id!: string;
  nickname?: string;
}

Branded / flavored types

The three matching-*-type rules compare the GraphQL scalar declared on the decorator (String, Int, ID, …) against the TypeScript annotation. By default the comparison is pure-AST, so a branded string like

type AIFDocumentId = Flavor<string, '__AIFDocumentId'>

would be treated as an unknown type — @Args({ type: () => ID }) id: AIFDocumentId would false-positive as a mismatch.

To fix this, enable typed linting by setting parserOptions.project (or projectService) in your ESLint config. When available, the rules use the TypeScript type checker to resolve the actual type — Flavor<string, …>, Brand<T, …>, intersections of string, aliases of aliases, etc. all reduce to their apparent primitive (string / number / boolean) and match the corresponding scalar.

Without typed linting, the rules fall back to the same AST-only comparison used in earlier versions — any non-primitive identifier that doesn't literally match the scalar name is flagged as a mismatch, which will false-positive on branded types. Enable typed linting to avoid this.

Installation

# inside your project's working tree
npm i eslint-plugin-nestjs-graphql --save-dev

The rules are off by default. To turn them on, add the following to your .eslintrc file:

{
  "plugins": ["nestjs-graphql"],
  "rules": {
    "nestjs-graphql/matching-return-type": "error", // `error` level is recommended
    "nestjs-graphql/matching-resolve-field-parent-type": "error", // `error` level is recommended
    "nestjs-graphql/matching-args-type": "error", // `error` level is recommended
    "nestjs-graphql/matching-field-type": "error", // `error` level is recommended
    "nestjs-graphql/require-resolve-field-for-nested-models": "error", // `error` level is recommended
    "nestjs-graphql/require-resolver-type-arg-with-resolve-field": "error", // `error` level is recommended
    "nestjs-graphql/no-optional-fields-in-object-type": "error", // `error` level is recommended
    "nestjs-graphql/no-redundant-field-decorator": "error", // compiler-plugin only; see rule docs
  }
}