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Resolver is a collection of functions that generate response for a GraphQL query. In simple terms, a resolver acts as a GraphQL query handler. Every resolver function in a GraphQL schema accepts four positional arguments as given below −

fieldName:(root, args, context, info) => { result }

An example of resolver functions is shown below −

//resolver function  with no parameters and returning string
greeting:() => {
   return "hello from  Adglob!!!"
}

//resolver function with no parameters and returning list
students:() => db.students.list()

//resolver function with arguments and returning object
studentById:(root,args,context,info) => {
   return db.students.get(args.id);
}

Given below are the positional arguments and their description −

Resolver Result Format

Resolvers in GraphQL can return different types of values as given below −

Illustration

Let us create a simple application to understand resolver. This will create schema for querying a student by id from the server. The student data will be stored in a flat file and we will use a node module called notarealdb to fake a database and read from flat file.

The following is a step-wise process to create a simple application −

Step 1 − Download and Install Required Dependencies for the Project

Create a folder named resolver-app. Change your directory to resolver-app from the terminal. Later, follow steps 3 to 5 in the Environment Setup chapter.

Step 2 − Create a Schema

Add schema.graphql file in the project folder resolver-app and add the following code −

type Query { 
   greeting:String
   students:[Student]
   studentById(id:ID!):Student 
}

type Student {
   id:ID!
   firstName:String
   lastName:String
   password:String
   collegeId:String
}

The schema file shows that user can query for greeting, students and studentById. To retrieve students with specific id, we use data type ID! which shows a non nullable unique identifier field. The students field returns an array of students, and greeting returns a simple string value.

Step 3 − Create Resolver

Create a file resolvers.js in the project folder and add the following code −

const db = require('./db')
const Query = {
   //resolver function for greeting
   greeting:() => {
      return "hello from  Adglob !!!"
   },
   
   //resolver function for students returns list
   students:() => db.students.list(),

   //resolver function for studentbyId
   studentById:(root,args,context,info) => {
      //args will contain parameter passed in query
      return db.students.get(args.id);
   }
}
module.exports = {Query}

Here, studentById takes in three parameters. As discussed in this chapter, the studentId can be retrieved from args; root will contain the Query object itself. To return a specific student, we need to call get method with id parameter in the students collection.

Here greeting, students, studentById are the resolvers that handle the query. students resolver function returns a list of students from the data access layer. To access resolver functions outside the module, Query object has to be exported using module.exports.

Step 4 − Run the Application

Create a server.js file. Refer step 8 in the Environment Setup Chapter. Execute the command npm start in the terminal. The server will be up and running on 9000 port. Here, we use GraphiQL as a client to test the application.

Open the browser and enter the url, http://localhost:9000/graphiql. Type the following query in the editor −

{  
   studentById(id:"S1001") {
      id
      firstName
      lastName
   }
}

The output for the above query is as shown below −

{
   "data": {
      "studentById": {
         "id": "S1001",
         "firstName": "Mohtashim",
         "lastName": "Mohammad"
      }
   }
}

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