Object types and input types in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple input type using NestJS GraphQL decorators.

NestJS

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

@InputType()
export class CreateUserInput {
  @Field()
  name: [1];
}

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

Aboolean

Bnumber

Cstring

DDate

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or boolean for a text field.

Forgetting to specify the type.

The 'name' field should be a string type to accept text input for the user's name.

2fill in blank

medium

Complete the code to add an optional email field to the input type.

NestJS

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

@InputType()
export class UpdateUserInput {
  @Field({ nullable: true })
  email?: [1];
}

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

Astring

Bnumber

Cboolean

DDate

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-string type for email.

Not marking the field as optional.

Email addresses are text, so the type should be string. The field is optional with '?' and nullable true.

3fill in blank

hard

Fix the error in the input type by completing the decorator to specify the field type explicitly.

NestJS

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

@InputType()
export class FilterUserInput {
  @Field(() => [1])
  isActive: boolean;
}

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

AString

BDate

CNumber

DBoolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using TypeScript types instead of GraphQL types in the decorator.

Omitting the type function in the decorator.

The @Field decorator needs the GraphQL Boolean type for a boolean field.

4fill in blank

hard

Fill both blanks to create an input type with a list of strings field.

NestJS

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

@InputType()
export class TagsInput {
  @Field(() => [[1]])
  tags: [2];
}

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

AString

Bstring[]

CBoolean

Dnumber[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using Boolean or number types instead of string.

Not matching GraphQL and TypeScript types.

The GraphQL type for a list of strings is [String], and the TypeScript type is string[].

5fill in blank

hard

Fill all three blanks to define an input type with street, city, and zipCode fields.

NestJS

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

@InputType()
export class AddressInput {
  @Field()
  street: [1];

  @Field()
  city: [2];

  @Field(() => [3])
  zipCode: number;
}

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

Astring

Bnumber

CInt

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing TypeScript and GraphQL types incorrectly.

Using lowercase 'number' in GraphQL decorator.

Street and city are text fields, so TypeScript type is string. zipCode is a number, so GraphQL type is Int.

Practice

1. What is the main purpose of using @ObjectType() in NestJS GraphQL?

easy

A. To define the shape of data returned by queries and mutations

B. To define the shape of data clients send to the server

C. To create database entities automatically

D. To handle HTTP request validation

Solution

1. Step 1: Understand the role of @ObjectType()

   This decorator is used to define the structure of data that GraphQL queries and mutations return to clients.

2. Step 2: Differentiate from input types

   @InputType() is used for data clients send, while @ObjectType() is for data sent back to clients.

3. Final Answer:

   To define the shape of data returned by queries and mutations -> Option A

4. Quick Check:

   ObjectType = output data shape [OK]

Hint: ObjectType decorates output data, InputType decorates input data [OK]

Common Mistakes:

• Confusing @ObjectType() with @InputType()
• Thinking @ObjectType() creates database tables
• Assuming @ObjectType() validates HTTP requests

2. Which of the following is the correct way to define an input type in NestJS GraphQL?

easy

A. @ObjectType() class CreateUserInput { name: string; }

B. @InputType() interface CreateUserInput { name: string; }

C. @InputType() class CreateUserInput { @Field() name: string; }

D. @Input() class CreateUserInput { name: string; }

Solution

1. Step 1: Identify correct decorator and syntax

   Input types require the @InputType() decorator and fields must be decorated with @Field() to expose them to GraphQL.

2. Step 2: Check class vs interface usage

   Input types must be classes, not interfaces, to work with decorators in NestJS.

3. Final Answer:

   @InputType() class CreateUserInput { @Field() name: string; } -> Option C

4. Quick Check:

   InputType + @Field + class = correct input type [OK]

Hint: Input types need @InputType() and @Field() on class properties [OK]

Common Mistakes:

• Using @ObjectType() instead of @InputType()
• Using interfaces instead of classes for input types
• Omitting @Field() decorator on fields

3. Given the following code, what will be the GraphQL output type for the user query?

@ObjectType()
class User {
  @Field()
  id: string;

  @Field()
  name: string;
}

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

medium

A. An error because id is not a number

B. A User object with fields id and name

C. A list of User objects

D. Null because no data source is connected

Solution

1. Step 1: Analyze the @ObjectType and @Field decorators

   The User class defines two fields, id and name, both exposed to GraphQL via @Field().

2. Step 2: Check the resolver return and query type

   The user query returns an object matching User shape with id and name strings, matching the GraphQL type.

3. Final Answer:

   A User object with fields id and name -> Option B

4. Quick Check:

   Query returns User object with id and name [OK]

Hint: Return object matches @ObjectType class shape for query output [OK]

Common Mistakes:

• Expecting type errors for string id when GraphQL expects string
• Assuming query returns a list without @Query(() => [User])
• Thinking null is returned without data source

4. Identify the error in this input type definition:

@InputType()
class UpdateUserInput {
  name: string;
  @Field()
  age: number;
}

medium

A. Missing @Field() decorator on the 'name' property

B. Cannot use number type in input types

C. Class must be decorated with @ObjectType() instead

D. Input types cannot have more than one property

Solution

1. Step 1: Check decorators on class properties

   In GraphQL input types, each field must have the @Field() decorator to be exposed.

2. Step 2: Identify missing decorator

   The 'name' property lacks the @Field() decorator, so it won't be part of the GraphQL input type.

3. Final Answer:

   Missing @Field() decorator on the 'name' property -> Option A

4. Quick Check:

   All input fields need @Field() [OK]

Hint: Every input field needs @Field() decorator [OK]

Common Mistakes:

• Forgetting @Field() on some properties
• Confusing @InputType() with @ObjectType()
• Thinking number types are invalid in input types

5. You want to create a mutation to update a user's profile with optional fields. Which approach correctly defines the input type to allow optional fields in NestJS GraphQL?

@InputType()
class UpdateUserInput {
  @Field({ nullable: true })
  name?: string;

  @Field({ nullable: true })
  age?: number;
}

hard

A. Use @ObjectType() instead of @InputType() for mutations

B. Remove '?' and set default values instead of nullable

C. Make all fields required and handle optional logic in resolver

D. Use @Field({ nullable: true }) and optional properties with '?' to allow partial updates

Solution

1. Step 1: Understand optional fields in GraphQL input types

   To allow optional fields, use { nullable: true } in @Field() and mark properties optional with '?'.

2. Step 2: Confirm correct decorator usage

   Use @InputType() for input data in mutations, not @ObjectType().

3. Final Answer:

   Use @Field({ nullable: true }) and optional properties with '?' to allow partial updates -> Option D

4. Quick Check:

   Nullable fields + optional properties = optional input fields [OK]

Hint: Use nullable:true and '?' for optional input fields [OK]

Common Mistakes:

• Using @ObjectType() for input types
• Not marking fields nullable for optional inputs
• Making all fields required and complicating resolver logic