What is the purpose of the @ nestjs /graphql package in Nest. Js ?

In the Nest.js framework, the @nestjs/graphql package is designed for building GraphQL APIs. GraphQL is a query language for APIs that enables clients to request precisely the data they need, rather than traditional REST APIs that may return unnecessary extra data.

Main Features

1. Define Schema: Using @nestjs/graphql, we can leverage decorators and TypeScript's type safety to define the GraphQL schema. For example, we can use the @ObjectType() decorator to define GraphQL types and @Field() to specify fields within those types.

   typescript
   @ObjectType()
   class User {
     @Field(type => Int)
     id: number;
   
     @Field()
     firstName: string;
   
     @Field()
     lastName: string;
   
     @Field()
     email: string;
   }

2. Resolvers: In Nest.js, resolvers handle queries for specific types or fields. Use the @Resolver() decorator to mark a class as a resolver. For example, create a UserResolver to manage data requests related to users.

   typescript
   @Resolver(of => User)
   export class UserResolver {
     constructor(private readonly userService: UserService) {}
   
     @Query(returns => User)
     async user(@Args('id') id: string): Promise<User> {
       return this.userService.findById(id);
     }
   }

3. Integration with Dependency Injection System: Similar to other components of Nest.js, @nestjs/graphql fully supports dependency injection, allowing you to inject services or providers into resolvers to manage business logic or database interactions.

4. Code-first and Schema-first Development Approaches: @nestjs/graphql supports two development approaches: Code-first and Schema-first. In the Code-first approach, you begin by writing TypeScript classes and decorators, and the framework then automatically generates the GraphQL schema for you. In the Schema-first approach, you start by defining the GraphQL schema, and then create the corresponding resolvers and classes based on it.

Example: User Query

Assume we need to implement a feature enabling clients to query user information. We can define a User type and a UserResolver class, and retrieve user data using GraphQL queries.

graphql
query {
  user(id: "124") {
    firstName
    lastName
    email
  }
}

In the above query, clients explicitly request the firstName, lastName, and email fields, and @nestjs/graphql simplifies handling such requests, making them efficient and straightforward.

In summary, the @nestjs/graphql package offers a powerful and flexible solution for building and managing GraphQL APIs in Nest.js, enabling developers to construct applications with type safety and modularity.