Resolver function signature in GraphQL

Introduction

A resolver function tells GraphQL how to get the data for a specific field. It connects the query to the actual data source.

When you want to fetch user details from a database after a query.

When you need to combine data from multiple sources for one field.

When you want to control how data is returned for a specific field.

When you want to add custom logic before sending data back.

When you want to handle errors or check permissions for data access.

Syntax

GraphQL

function resolver(parent, args, context, info) {
  // return data
}

parent: The result from the previous resolver or root object.

args: The arguments passed to the field in the query.

context: Shared data like authentication info or database connections.

info: Information about the execution state of the query.

Examples

This resolver fetches a user by ID from the database.

GraphQL

function getUser(parent, args, context, info) {
  return context.db.findUserById(args.id);
}

This resolver modifies a field by changing the parent's name to uppercase.

GraphQL

const resolver = (parent, args, context) => {
  return parent.name.toUpperCase();
};

Sample Program

This simple resolver returns a greeting string when the 'hello' query is called.

GraphQL

const resolvers = {
  Query: {
    hello: (parent, args, context, info) => {
      return 'Hello, world!';
    }
  }
};

Important Notes

Resolvers run in the order fields are requested in the query.

If a resolver is missing for a field, GraphQL returns the value of that field from the parent object.

Summary

Resolver functions connect queries to data sources.

They receive four arguments: parent, args, context, and info.

Resolvers can return data directly or perform logic before returning.