What is a resolver in NestJS GraphQL and how do you define a root query resolver?

Q: What is a resolver in NestJS GraphQL and how do you define a root query resolver?

@Resolver(() => User) export class UsersResolver { constructor(private usersService: UsersService) {} // Maps to: type Query { users: [User!]! } @Query(() => [User], { name: 'users' }) findAll(): Promise { return this.usersService.findAll(); } // Maps to: type Query { user(id: Int!): User } @Query(() => User, { nullable: true, name: 'user' }) findOne( @Args('id', { type: () => Int }) id: number, ): Promise { return this.usersService.findOne(id); } } @Resolver(() => User) — tells NestJS this class handles field resolution for the User type. @Query(() => ReturnType) — the arrow function is required; TypeScript metadata cannot represent GraphQL arrays. name option in @Query() — overrides the default field name; otherwise the method name is used. nullable: true — marks the field as returning User | null in the generated schema. Resolvers are registered in the module's providers array like any other NestJS provider.

Difficulty: 5/10

GraphQL resolvers, NestJS decorators, Root query definition

A resolver is a class decorated with @Resolver() that maps GraphQL operations to service calls. Each method decorated with @Query(), @Mutation(), or @Subscription() maps to a field in the schema's root types. The arrow function return type in @Query() is required because TypeScript metadata cannot represent GraphQL array types.

Root query resolver definition

Resolver key facts:

1
@Resolver(() => User) — tells NestJS this class handles field resolution for the User type.
2
@Query(() => ReturnType) — the arrow function is required; TypeScript metadata cannot represent GraphQL arrays.
3
name option in @Query() — overrides the default field name; otherwise the method name is used.
4
nullable: true — marks the field as returning User | null in the generated schema.
5
Resolvers are registered in the module's providers array like any other NestJS provider.

Scenario Questions

0-2 years experience

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