Resolvers in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this GraphQL resolver method?

Consider this NestJS GraphQL resolver method:

@Query(() => String)
getGreeting() {
  return `Hello, NestJS!`;
}

What will be the response when this query is called?

NestJS

@Query(() => String)
getGreeting() {
  return `Hello, NestJS!`;
}

A "getGreeting"

B null

C An error because of missing async

D "Hello, NestJS!"

💡 Hint

Look at the return value of the method and its type.

The resolver method returns the string "Hello, NestJS!" directly. The GraphQL query will respond with this string.

❓ state_output

intermediate

What value does this resolver return after state change?

Given this resolver snippet:

@Resolver()
export class CounterResolver {
  private count = 0;

  @Query(() => Number)
  getCount() {
    return this.count;
  }

  @Mutation(() => Number)
  increment() {
    this.count++;
    return this.count;
  }
}

If the increment mutation is called twice, what will getCount return?

NestJS

@Resolver()
export class CounterResolver {
  private count = 0;

  @Query(() => Number)
  getCount() {
    return this.count;
  }

  @Mutation(() => Number)
  increment() {
    this.count++;
    return this.count;
  }
}

A 1

B 0

C 2

D undefined

💡 Hint

Think about how many times the count is increased and what the initial value is.

The count starts at 0. Each call to increment adds 1. After two calls, count is 2, so getCount returns 2.

📝 Syntax

advanced

Which option correctly defines a resolver method with arguments?

Which of the following resolver methods correctly accepts an argument id of type string and returns it?

A

@Query(() => String)
getById(@Args('id') id: string) {
  return id;
}

B

@Query(() => String)
getById(id: string) {
  return id;
}

C

@Query(() => String)
getById(@Arg('id') id: string) {
  return id;
}

D

@Query(() => String)
getById(@Args id: string) {
  return id;
}

💡 Hint

Remember the NestJS decorator to extract GraphQL arguments.

In NestJS GraphQL, @Args('id') is used to get the argument named 'id'. Other options use incorrect decorators or syntax.

🔧 Debug

advanced

What error does this resolver code produce?

Consider this resolver code snippet:

@Query(() => String)
async fetchData() {
  const data = await this.service.getData();
  return data.value;
}

If this.service.getData() returns null, what error will occur?

NestJS

@Query(() => String)
async fetchData() {
  const data = await this.service.getData();
  return data.value;
}

A SyntaxError: Unexpected token 'await'

B TypeError: Cannot read property 'value' of null

C ReferenceError: service is not defined

D No error, returns undefined

💡 Hint

What happens if you try to access a property on null?

Accessing data.value when data is null causes a TypeError because null has no properties.

🧠 Conceptual

expert

Which statement best describes resolver execution order in NestJS GraphQL?

In NestJS GraphQL, when a query requests multiple fields resolved by different resolver methods, how does NestJS execute these resolvers?

A Resolvers execute in parallel for fields at the same level, but nested fields wait for parent resolvers.

B All resolvers execute sequentially regardless of nesting or field level.

C Resolvers execute strictly one after another in the order they are defined in the code.

D Resolvers execute randomly without any predictable order.

💡 Hint

Think about how GraphQL resolves fields and nested fields.

GraphQL executes resolvers for fields at the same level in parallel. Nested fields wait for their parent field's resolver to complete before executing.