Schema-first approach in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS GraphQL schema-first resolver?

Given this resolver method, what will be the returned value when the query hello is called?

NestJS

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

@Resolver()
export class HelloResolver {
  @Query(() => String)
  hello() {
    return 'Hello, Schema-first!';
  }
}

A Throws a runtime error

B null

C undefined

D "Hello, Schema-first!"

💡 Hint

Look at the return statement inside the resolver method.

The resolver method hello returns the string 'Hello, Schema-first!'. When the GraphQL query hello is called, this string is returned as the response.

📝 Syntax

intermediate

Which option correctly defines a GraphQL schema type in schema-first approach?

Choose the valid GraphQL schema type definition for a User with fields id (ID!) and name (String).

A type User { id: ID! name: String }

B type User { id: ID name: String! }

C type User { id: ID! name: String! }

D type User { id: ID! name: Int }

💡 Hint

Check the required fields and their types carefully.

Option A correctly defines id as a non-nullable ID and name as a nullable String, matching the prompt exactly.

🔧 Debug

advanced

What error does this NestJS schema-first resolver code produce?

Identify the error when running this resolver code:

NestJS

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

@Resolver()
export class UserResolver {
  @Query(() => User)
  getUser() {
    return { id: 1, name: 'Alice' };
  }
}

class User {
  id: string;
  name: string;
}

A TypeError: Cannot read property 'id' of undefined

B GraphQL error: Expected type String, found Int

C SyntaxError: Unexpected token class

D No error, returns user object correctly

💡 Hint

Check the type of the id field in the returned object versus the schema class.

The resolver returns id as a number (1), but the schema expects a string. GraphQL throws an error because it found an Int where a String was expected.

❓ state_output

advanced

What is the value of result after executing this NestJS schema-first query resolver?

Consider this resolver method and the query call getNumbers:

NestJS

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

@Resolver()
export class NumberResolver {
  @Query(() => [Int])
  getNumbers() {
    return [1, 2, 3].map(x => x * 2);
  }
}

// Query: getNumbers

A [2, 4, 6]

B Throws a runtime error

C [0, 2, 4]

D [1, 2, 3]

💡 Hint

Look at the map function multiplying each number by 2.

The array [1, 2, 3] is mapped to a new array where each element is doubled, resulting in [2, 4, 6].

🧠 Conceptual

expert

Which statement best describes the schema-first approach in NestJS GraphQL?

Choose the most accurate description of the schema-first approach.

A Resolvers are written without any schema, and schema is inferred at runtime.

B You write TypeScript classes first, and the schema is generated automatically from them.

C You write GraphQL schema definitions first, then implement resolvers to match the schema.

D Schema and resolvers are written together in a single file using decorators only.

💡 Hint

Think about the order of writing schema and resolvers in schema-first.

Schema-first means you define the GraphQL schema first (usually in .graphql files), then write resolvers to implement the schema.