Code-first approach in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a GraphQL resolver using the code-first approach in NestJS.

NestJS

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

@Resolver()
export class SampleResolver {
  @Query(() => String)
  getHello() {
    return [1];
  }
}

Drag options to blanks, or click blank then click option'

A'Hello World!'

BHello World!

Creturn 'Hello World!'

Dconsole.log('Hello World!')

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string

Using console.log instead of returning a value

The query method should return a string value. Using a string literal like 'Hello World!' is correct.

2fill in blank

medium

Complete the code to define a GraphQL object type using the code-first approach.

NestJS

import { ObjectType, Field, Int } from '@nestjs/graphql';

@ObjectType()
export class [1] {
  @Field()
  name: string;

  @Field(() => Int)
  age: number;
}

Drag options to blanks, or click blank then click option'

AMutation

BUser

CResolver

DQuery

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Query' or 'Mutation' as object type names

Using decorator names as class names

The class name should represent the GraphQL object type. 'User' is a common example.

3fill in blank

hard

Fix the error in the code to correctly define a mutation using the code-first approach.

NestJS

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

@Resolver()
export class SampleResolver {
  @Mutation(() => Boolean)
  async updateName(@Args('name') [1]: string) {
    // update logic here
    return true;
  }
}

Drag options to blanks, or click blank then click option'

Aname

BName

CnewName

DupdateName

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase or different names than the @Args string

Mismatching argument names

The argument name in @Args decorator and the parameter name should match exactly and be lowercase.

4fill in blank

hard

Fill both blanks to create a GraphQL input type and use it in a mutation.

NestJS

import { InputType, Field } from '@nestjs/graphql';

@[1]()
export class UpdateUserInput {
  @Field()
  name: string;
}

@Resolver()
export class UserResolver {
  @Mutation(() => Boolean)
  async updateUser(@Args('[2]') input: UpdateUserInput) {
    // update logic
    return true;
  }
}

Drag options to blanks, or click blank then click option'

AInputType

BUpdateUserInput

CMutationInput

DUserInput

Attempts:

3 left

💡 Hint

Common Mistakes

Using @ObjectType instead of @InputType

Mismatching argument names in @Args

The class should be decorated with @InputType. The argument name in @Args should match the input class name.

5fill in blank

hard

Fill all three blanks to define a GraphQL resolver method that returns a list of users with proper decorators.

NestJS

import { Resolver, Query } from '@nestjs/graphql';
import { User } from './user.model';

@Resolver(() => User)
export class UserResolver {
  @Query(() => [[1]])
  async [2]() : Promise<[3][]> {
    // fetch users logic
    return [];
  }
}

Drag options to blanks, or click blank then click option'

AUser

Busers

DUserEntity

Attempts:

3 left

💡 Hint

Common Mistakes

Using singular method names for list queries

Mismatching return types and decorator types

The query returns a list of User objects. The method name is typically plural like 'users'. The return type is Promise of User array.

Practice

1. What does the code-first approach in NestJS GraphQL primarily involve?

easy

A. Manually writing GraphQL schema files before coding

B. Using REST endpoints instead of GraphQL

C. Writing TypeScript classes and decorators to define GraphQL schema

D. Generating code from existing GraphQL schema files

Solution

1. Step 1: Understand code-first approach basics

   The code-first approach means you write your GraphQL types and resolvers directly in TypeScript code using classes and decorators.

2. Step 2: Compare with other approaches

   Unlike schema-first, you do not write schema files manually; instead, the schema is generated automatically from your code.

3. Final Answer:

   Writing TypeScript classes and decorators to define GraphQL schema -> Option C

4. Quick Check:

   Code-first = Define schema in code [OK]

Hint: Code-first means schema comes from your TypeScript classes [OK]

Common Mistakes:

• Confusing code-first with schema-first approach
• Thinking schema files must be manually written
• Assuming REST is involved in code-first GraphQL

2. Which decorator is used in NestJS code-first approach to mark a class as a GraphQL object type?

easy

A. @ObjectType()

B. @Resolver()

C. @Query()

D. @Injectable()

Solution

1. Step 1: Identify decorator for GraphQL object type

   In code-first, @ObjectType() marks a class as a GraphQL object type to generate schema.

2. Step 2: Differentiate from other decorators

   @Resolver() marks resolver classes, @Query() marks query methods, and @Injectable() is for dependency injection, not schema types.

3. Final Answer:

   @ObjectType() -> Option A

4. Quick Check:

   @ObjectType = GraphQL type class [OK]

Hint: Use @ObjectType() for GraphQL types, not @Resolver() [OK]

Common Mistakes:

• Using @Resolver() instead of @ObjectType() for types
• Confusing @Query() with type decorators
• Thinking @Injectable() defines GraphQL schema

3. Given this code-first resolver snippet, what will be the GraphQL query result for hello()?

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

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

medium

A. "Hello, NestJS!"

B. null

C. Error: Missing return type

D. undefined

Solution

1. Step 1: Analyze the resolver method

   The method hello() returns the string 'Hello, NestJS!'. The @Query decorator specifies the return type as String.

2. Step 2: Understand GraphQL query result

   When queried, the hello field returns the string value wrapped in quotes as a valid GraphQL string response.

3. Final Answer:

   "Hello, NestJS!" -> Option A

4. Quick Check:

   hello() returns string 'Hello, NestJS!' [OK]

Hint: Resolver method return matches GraphQL query result [OK]

Common Mistakes:

• Expecting null or undefined instead of returned string
• Thinking missing return type causes runtime error
• Confusing resolver method with schema definition

4. Identify the error in this code-first resolver snippet:

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

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

medium

A. Method sample() should be async

B. Missing return type in @Query decorator

C. Resolver class missing @ObjectType decorator

D. Return value 123 is invalid in GraphQL

Solution

1. Step 1: Check @Query decorator usage

   The @Query decorator requires a return type function, e.g., @Query(() => Int) or @Query(() => String).

2. Step 2: Identify missing return type

   Here, @Query() is missing the return type, which causes schema generation errors.

3. Final Answer:

   Missing return type in @Query decorator -> Option B

4. Quick Check:

   @Query needs return type function [OK]

Hint: Always specify return type in @Query(() => Type) [OK]

Common Mistakes:

• Omitting return type in @Query decorator
• Adding @ObjectType to resolver class incorrectly
• Assuming async is mandatory for resolver methods

5. You want to create a GraphQL object type with a field that is optional and nullable using the code-first approach. Which is the correct way to define the field in your class?

import { Field, ObjectType } from '@nestjs/graphql';

@ObjectType()
export class User {
  @Field(() => String, { nullable: true })
  nickname?: string;
}

What does this code do?

hard

A. Ignores the nickname field in the generated schema

B. Causes a schema error because optional fields are not allowed

C. Makes nickname required and non-nullable in the schema

D. Defines a User type with an optional nullable nickname field in GraphQL schema

Solution

1. Step 1: Understand @Field decorator options

   The @Field decorator with { nullable: true } marks the field as nullable in GraphQL schema, allowing null values.

2. Step 2: Check TypeScript optional property

   Using ? in nickname?: string means the property is optional in TypeScript, matching nullable in GraphQL.

3. Step 3: Confirm schema generation behavior

   This combination correctly generates a GraphQL field that can be omitted or set to null without errors.

4. Final Answer:

   Defines a User type with an optional nullable nickname field in GraphQL schema -> Option D

5. Quick Check:

   nullable + optional = optional nullable field [OK]

Hint: Use @Field({ nullable: true }) and ? for optional nullable fields [OK]

Common Mistakes:

• Omitting nullable option for optional fields
• Confusing optional in TypeScript with required in GraphQL
• Expecting errors for optional nullable fields