Queries and mutations in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS GraphQL query resolver?

Consider this NestJS GraphQL query resolver code. What will be the output when the query getGreeting is called?

NestJS

import { Query, Resolver } from '@nestjs/graphql';

@Resolver()
export class GreetingResolver {
  @Query(() => String)
  getGreeting(): string {
    return 'Hello, NestJS!';
  }
}

A "Hello, NestJS!"

B undefined

C null

D Error: Missing return type

💡 Hint

Look at the return value inside the getGreeting method.

The getGreeting method returns the string 'Hello, NestJS!'. The GraphQL query will return this exact string.

📝 Syntax

intermediate

Which option correctly defines a mutation to add a user in NestJS GraphQL?

You want to create a mutation named addUser that accepts a name string and returns a boolean. Which code snippet is correct?

A

@Mutation(() => Boolean)
addUser(@Args('name') name: string): boolean {
  return true;
}

B

@Mutation(() => Boolean)
addUser(name: string): boolean {
  return true;
}

C

@Mutation()
addUser(name: string): boolean {
  return true;
}

D

@Query(() => Boolean)
addUser(@Args('name') name: string): boolean {
  return true;
}

💡 Hint

Mutations use the @Mutation decorator and arguments must be decorated with @Args.

Option A correctly uses @Mutation with a return type and decorates the argument with @Args. Other options either use @Query or miss @Args decorator.

❓ state_output

advanced

What is the value of userCount after these mutations?

Given this NestJS resolver code, what is the value of userCount after calling addUser twice?

NestJS

import { Mutation, Resolver } from '@nestjs/graphql';

@Resolver()
export class UserResolver {
  private userCount = 0;

  @Mutation(() => Number)
  addUser(): number {
    this.userCount += 1;
    return this.userCount;
  }
}

A Error: userCount is private

B 1

C 2

D 0

💡 Hint

Each call to addUser increases userCount by 1 and returns it.

The private variable userCount starts at 0. Each mutation call increments it by 1. After two calls, userCount is 2.

🔧 Debug

advanced

What error does this NestJS GraphQL mutation code produce?

This mutation is intended to update a user's email but causes an error. What error is raised?

NestJS

import { Mutation, Args, Resolver } from '@nestjs/graphql';

@Resolver()
export class UserResolver {
  @Mutation(() => Boolean)
  updateEmail(@Args('email') email: string): boolean {
    this.email = email;
    return true;
  }
}

A No error, returns true

B SyntaxError: Unexpected token

C ReferenceError: email is not defined

D TypeScript error: Property 'email' does not exist on type 'UserResolver'

💡 Hint

Check if 'this.email' is defined in the class.

The class does not define 'email' property, so 'this.email = email' causes a TypeScript compile error: Property 'email' does not exist on type 'UserResolver'.

🧠 Conceptual

expert

Which statement best describes the difference between queries and mutations in NestJS GraphQL?

Choose the most accurate description of queries and mutations in NestJS GraphQL.

A Queries modify data; mutations only fetch data without side effects.

B Queries fetch data without side effects; mutations modify data and may cause side effects.

C Both queries and mutations only fetch data but differ in syntax.

D Mutations are faster than queries because they run on the server.

💡 Hint

Think about what each operation does to data.

Queries are for reading data without changing it. Mutations change data and can have side effects like updating a database.