Resolvers in NestJS - Step-by-Step Execution

Concept Flow - Resolvers

Client sends GraphQL query

↓

GraphQL Module receives query

↓

Resolver method matched to query

↓

Resolver executes business logic

↓

Resolver returns data

↓

GraphQL Module sends response to client

Resolvers receive GraphQL queries, run the matching method, and return data back to the client.

Execution Sample

NestJS

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

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

This resolver responds to the 'sayHello' query by returning a greeting string.

Execution Table

Step	Action	Input	Resolver Method	Output
1	Receive query	{ sayHello }	N/A	N/A
2	Match query to resolver	sayHello	sayHello()	N/A
3	Execute resolver method	N/A	sayHello()	'Hello, world!'
4	Send response	N/A	N/A	{ "sayHello": "Hello, world!" }

💡 Query fully resolved and response sent to client.

Variable Tracker

Variable	Start	After Step 3	Final
query	N/A	{ sayHello }	{ sayHello }
resolverMethod	N/A	sayHello()	sayHello()
result	N/A	N/A	'Hello, world!'

Key Moments - 2 Insights

Why does the resolver method name match the query name?

The GraphQL query name must match the resolver method decorated with @Query to connect the request to the correct function, as shown in step 2 of the execution table.

What happens if the resolver method does not return data?

If the resolver returns nothing or undefined, the client will receive null or an error. Step 3 shows the resolver returning a string, which is required for a successful response.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 3?

A { sayHello }

B N/A

C 'Hello, world!'

D null

💡 Hint

Check the 'Output' column at step 3 in the execution table.

At which step does the resolver method get matched to the query?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column describing matching in the execution table.

If the resolver method returned null, how would the final output change?

A Output would be an error at step 2

B Output would be { 'sayHello': null }

C Output would be { 'sayHello': 'Hello, world!' }

D No response would be sent

💡 Hint

Refer to the 'Output' column and understand what happens when resolver returns null.

Concept Snapshot

Resolvers connect GraphQL queries to methods.
Use @Resolver() class and @Query() method decorators.
Method name matches query name.
Return data from method to send response.
NestJS handles query routing automatically.

Full Transcript

Resolvers in NestJS handle GraphQL queries by matching the query name to a method decorated with @Query inside a class decorated with @Resolver. When a client sends a query, the GraphQL module finds the matching resolver method, executes it, and returns the data. For example, a 'sayHello' query calls the 'sayHello' method, which returns 'Hello, world!'. The response is then sent back to the client. This flow ensures queries get the correct data by connecting query names to resolver methods.