Object types and input types in NestJS - Mini Project: Build & Apply

NestJS Object Types and Input Types

📖 Scenario: You are building a simple NestJS API to manage a list of books in a library. Each book has a title and an author.We will create object types to represent the book data and input types to handle new book creation requests.

🎯 Goal: Create a NestJS GraphQL object type for Book and an input type for CreateBookInput. Then use these types in a resolver to add new books.

📋 What You'll Learn

Create a GraphQL object type called Book with fields title and author of type string.

Create a GraphQL input type called CreateBookInput with fields title and author of type string.

Create a resolver method called addBook that takes CreateBookInput as an argument and returns a Book.

Use decorators @ObjectType(), @Field(), and @InputType() correctly.

💡 Why This Matters

🌍 Real World

APIs often need to define clear data structures for input and output. NestJS GraphQL uses object types and input types to do this cleanly.

💼 Career

Understanding how to create and use object and input types in NestJS GraphQL is essential for backend developers working with modern APIs.

1

Create the Book object type

Create a class called Book decorated with @ObjectType(). Add two string fields: title and author, each decorated with @Field().

NestJS

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

// Define the Book object type here
# Your code here

Need a hint?

Use @ObjectType() above the class and @Field() above each property.

2

Create the CreateBookInput input type

Create a class called CreateBookInput decorated with @InputType(). Add two string fields: title and author, each decorated with @Field().

NestJS

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

@ObjectType()
export class Book {
  @Field()
  title: string;

  @Field()
  author: string;
}

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

// Define the CreateBookInput input type here
# Your code here

Need a hint?

Use @InputType() above the class and @Field() above each property.

3

Create the addBook resolver method

In a resolver class called BooksResolver, create a method called addBook decorated with @Mutation(() => Book). It should take one argument called createBookInput of type CreateBookInput decorated with @Args('createBookInput'). The method should return a new Book object with the same title and author as createBookInput.

NestJS

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

@ObjectType()
export class Book {
  @Field()
  title: string;

  @Field()
  author: string;
}

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

@InputType()
export class CreateBookInput {
  @Field()
  title: string;

  @Field()
  author: string;
}

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

@Resolver()
export class BooksResolver {
  // Create addBook mutation here
  # Your code here
}

Need a hint?

Use @Mutation(() => Book) above the method and @Args('createBookInput') for the argument.

4

Export the resolver and complete setup

Export the BooksResolver class and ensure all imports are correct. This completes the setup for object types and input types in NestJS GraphQL.

NestJS

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

@ObjectType()
export class Book {
  @Field()
  title: string;

  @Field()
  author: string;
}

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

@InputType()
export class CreateBookInput {
  @Field()
  title: string;

  @Field()
  author: string;
}

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

@Resolver()
export class BooksResolver {
  @Mutation(() => Book)
  addBook(@Args('createBookInput') createBookInput: CreateBookInput): Book {
    return {
      title: createBookInput.title,
      author: createBookInput.author
    };
  }
}

// Export the resolver class
# Your code here

Need a hint?

Use export { BooksResolver }; at the end of the file.