Resolvers in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Resolver in NestJS GraphQL?

A Resolver is a special class in NestJS that handles GraphQL queries and mutations. It connects GraphQL requests to the code that fetches or modifies data.

beginner

How do you define a Query in a NestJS Resolver?

You use the @Query() decorator on a method inside a Resolver class. This method runs when the GraphQL query with the same name is called.

beginner

What decorator is used to create a Mutation in a NestJS Resolver?

The @Mutation() decorator is used on a method inside a Resolver class to handle GraphQL mutations, which change data.

intermediate

Why do Resolvers use dependency injection in NestJS?

Resolvers use dependency injection to get services that handle the actual data logic. This keeps the Resolver focused on GraphQL and clean code separation.

beginner

What is the role of the @Resolver() decorator in NestJS?

The @Resolver() decorator marks a class as a GraphQL Resolver. It can optionally take an object type to link the Resolver to a GraphQL type.

Which decorator defines a GraphQL query method in a NestJS Resolver?

A @Injectable()

B @Mutation()

C @Resolver()

D @Query()

The @Query() decorator marks a method as a GraphQL query handler.

What does the @Resolver() decorator do?

A Defines a GraphQL mutation

B Marks a class as a GraphQL Resolver

C Injects a service into a class

D Creates a GraphQL schema

The @Resolver() decorator marks a class as a GraphQL Resolver in NestJS.

How do Resolvers get access to services in NestJS?

A Using dependency injection in the constructor

B By importing them directly

C Using global variables

D By calling static methods

Resolvers receive services via dependency injection through their constructor.

Which decorator is used to handle data changes in GraphQL with NestJS?

A @Mutation()

B @Resolver()

C @Query()

D @Controller()

The @Mutation() decorator is used for GraphQL mutations that change data.

What is the main purpose of a Resolver in NestJS GraphQL?

A To define REST endpoints

B To manage database connections

C To connect GraphQL requests to data logic

D To style the frontend

Resolvers connect GraphQL queries and mutations to the code that fetches or changes data.

Explain how to create a simple Query Resolver method in NestJS and what it does.

Think about how a method is marked to respond to a GraphQL query.

Describe the role of dependency injection in NestJS Resolvers and why it is useful.

Consider how services are provided to Resolvers without manual creation.

Practice

1. What is the main purpose of a @Resolver in a NestJS GraphQL application?

easy

A. To handle GraphQL queries and mutations by connecting them to data or actions

B. To define database schemas for the application

C. To manage HTTP routes and middleware

D. To style the frontend components

Solution

1. Step 1: Understand the role of @Resolver

   The @Resolver decorator marks a class that handles GraphQL queries and mutations, linking them to the app's data or business logic.

2. Step 2: Differentiate from other roles

   It does not define database schemas, manage HTTP routes, or style frontend components, which are handled by other parts of the app.

3. Final Answer:

   To handle GraphQL queries and mutations by connecting them to data or actions -> Option A

4. Quick Check:

   Resolver purpose = handle GraphQL queries/mutations [OK]

Hint: Resolvers link GraphQL requests to your app's data/actions [OK]

Common Mistakes:

• Confusing resolvers with database schema definitions
• Thinking resolvers manage HTTP routes
• Assuming resolvers handle frontend styling

2. Which of the following is the correct way to define a GraphQL query resolver method in NestJS?

easy

A. @Controller()
getHello() {
return 'Hello World';
}

B. @Mutation(() => String)
getHello() {
return 'Hello World';
}

C. @Query(() => String)
getHello() {
return 'Hello World';
}

D. @Resolver(() => String)
getHello() {
return 'Hello World';
}

Solution

1. Step 1: Identify the decorator for queries

   In NestJS GraphQL, @Query decorator is used to define query resolver methods.

2. Step 2: Check method syntax

   The method returns a string and is decorated with @Query(() => String), which is correct syntax.

3. Final Answer:

   @Query(() => String) getHello() { return 'Hello World'; } -> Option C

4. Quick Check:

   Query method uses @Query decorator [OK]

Hint: Use @Query for queries, @Mutation for mutations [OK]

Common Mistakes:

• Using @Mutation instead of @Query for queries
• Using @Resolver on methods instead of classes
• Using @Controller which is for REST, not GraphQL

3. Given this resolver code, what will be the output when the hello query is called?

@Resolver()
export class HelloResolver {
  @Query(() => String)
  hello() {
    return 'Hi NestJS';
  }
}

medium

A. Error: Missing return type

B. "Hello World"

C. null

D. "Hi NestJS"

Solution

1. Step 1: Analyze the resolver method

   The hello method is decorated with @Query(() => String) and returns the string 'Hi NestJS'.

2. Step 2: Determine the query output

   When the hello query is called, it returns the string exactly as coded: 'Hi NestJS'.

3. Final Answer:

   "Hi NestJS" -> Option D

4. Quick Check:

   Query returns 'Hi NestJS' string [OK]

Hint: Return value inside @Query method is the output [OK]

Common Mistakes:

• Expecting 'Hello World' instead of 'Hi NestJS'
• Thinking a return type error occurs
• Assuming null is returned by default

4. Identify the error in this NestJS resolver code:

@Resolver()
export class SampleResolver {
  @Query(() => String)
  sample() {
    return 123;
  }
}

medium

A. The method returns a number but the return type is declared as String

B. Missing @Mutation decorator

C. The class is missing @Injectable decorator

D. The method name should be capitalized

Solution

1. Step 1: Check the return type annotation

   The @Query(() => String) decorator declares the return type as String.

2. Step 2: Check the actual return value

   The method returns the number 123, which conflicts with the declared String return type.

3. Final Answer:

   The method returns a number but the return type is declared as String -> Option A

4. Quick Check:

   Return type mismatch causes error [OK]

Hint: Return type must match decorator type [OK]

Common Mistakes:

• Ignoring return type mismatch
• Thinking @Mutation is needed for queries
• Assuming @Injectable is required on resolvers

5. You want to create a resolver that returns a list of users filtered by an optional age argument. Which code snippet correctly implements this in NestJS?

hard

A. @Resolver()
export class UserResolver {
@Mutation(() => [User])
users(@Args('age') age: number) {
return users.filter(u => u.age === age);
}
}

B. @Resolver()
export class UserResolver {
@Query(() => [User])
users(@Args('age', { nullable: true }) age?: number) {
return age ? users.filter(u => u.age === age) : users;
}
}

C. @Resolver()
export class UserResolver {
@Query(() => User)
users(@Args('age') age: number) {
return users.find(u => u.age === age);
}
}

D. @Resolver()
export class UserResolver {
@Query(() => [User])
users() {
return users;
}
}

Solution

1. Step 1: Check the query decorator and return type

   The method uses @Query(() => [User]) which correctly indicates it returns a list of users.

2. Step 2: Verify optional argument handling

   The @Args('age', { nullable: true }) age?: number allows the age argument to be optional, filtering users if provided, or returning all users otherwise.

3. Step 3: Confirm filtering logic

   The method filters the users array by age if age is given, otherwise returns all users, matching the requirement.

4. Final Answer:

   @Resolver() export class UserResolver { @Query(() => [User]) users(@Args('age', { nullable: true }) age?: number) { return age ? users.filter(u => u.age === age) : users; } } -> Option B

5. Quick Check:

   @Resolver() export class UserResolver { @Query(() => [User]) users(@Args('age', { nullable: true }) age?: number) { return age ? users.filter(u => u.age === age) : users; } } [OK]

Hint: Use nullable args for optional filters in @Query [OK]

Common Mistakes:

• Using @Mutation instead of @Query for fetching data
• Returning single User instead of list
• Not making argument optional when needed