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Javascript NestJS Error: Cannot determine a GraphQL output type
Understanding NestJS and GraphQL
NestJS is a progressive Node.js framework for building efficient, reliable, and scalable server-side applications. It leverages TypeScript and combines elements of OOP (Object Oriented Programming), FP (Functional Programming), and FRP (Functional Reactive Programming). One of its powerful features is the ability to integrate with GraphQL, a query language for APIs that allows clients to request only the data they need.
Identifying the Symptom
When working with NestJS and GraphQL, you might encounter the error: Error: Cannot determine a GraphQL output type. This error typically arises when there is a mismatch or misconfiguration in the GraphQL schema or resolver.
Exploring the Issue
The error Cannot determine a GraphQL output type indicates that the GraphQL resolver is not returning a valid output type. This could be due to several reasons, such as:
- The resolver function is not returning the expected data type.
- The GraphQL schema is not correctly defined or is missing type definitions.
- There is a mismatch between the resolver return type and the schema type definition.
Common Causes
One common cause is forgetting to annotate the return type of a resolver function. Another is using a type that GraphQL cannot map to a known GraphQL type.
Steps to Fix the Issue
Step 1: Verify the Resolver Return Type
Ensure that your resolver functions return a valid GraphQL type. If you are using TypeScript, make sure to explicitly define the return type of your resolver functions. For example:
import { Resolver, Query } from '@nestjs/graphql';
import { MyType } from './my-type.model';
@Resolver(of => MyType)
export class MyResolver {
@Query(returns => MyType)
getMyType(): MyType {
return { /* valid MyType object */ };
}
}
Step 2: Check Your GraphQL Schema
Ensure that your GraphQL schema is correctly defined. You can use tools like GraphQL Schema Definition Language (SDL) to define your types. Make sure all types used in resolvers are declared in the schema.
Step 3: Synchronize Types
Ensure that the types defined in your TypeScript models match those in your GraphQL schema. Use decorators like @ObjectType and @Field from @nestjs/graphql to map TypeScript classes to GraphQL types:
import { ObjectType, Field, Int } from '@nestjs/graphql';
@ObjectType()
export class MyType {
@Field(type => Int)
id: number;
@Field()
name: string;
}
Step 4: Validate with GraphQL Tools
Use tools like GraphQL Playground or Postman to test your queries and ensure that the schema and resolvers are correctly configured.
Conclusion
By following these steps, you should be able to resolve the Cannot determine a GraphQL output type error in your NestJS application. Ensuring that your resolvers return valid types and that your schema is correctly defined are key to leveraging the full power of GraphQL with NestJS.
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