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

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);
    }
  }

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
  }
}