server.js

var express = require('express');
var { graphqlHTTP } = require('express-graphql');
var { buildSchema } = require('graphql');

// Initialize a GraphQL schema
var schema = buildSchema(`
  type Query {
    user(id: Int!): Person
    users(shark: String): [Person]
  },
  type Person {
    id: Int
    name: String
    age: Int
    shark: String
  }
  type Mutation {
    updateUser(id: Int!, name: String!, age: String): Person
  }
`);
  // Sample users
  var users = [
    {
      id: 1,
      name: 'Brian',
      age: '21',
      shark: 'Great White Shark'
    },
    {
      id: 2,
      name: 'Kim',
      age: '22',
      shark: 'Whale Shark'
    },
    {
      id: 3,
      name: 'Faith',
      age: '23',
      shark: 'Hammerhead Shark'
    },
    {
      id: 4,
      name: 'Joseph',
      age: '23',
      shark: 'Tiger Shark',
    },
    {
      id: 5,
      name: 'Joy',
      age: '25',
      shark: 'Hammerhead Shark'
    }
  ];

  // Return a single user (based on id)
var getUser = function(args) {
  var userID = args.id;
  return users.filter(user => user.id == userID)[0];
}

// Return a list of users (takes an optional shark parameter)
var retrieveUsers = function(args) {
  if (args.shark) {
    var shark = args.shark;
    return users.filter(user => user.shark == shark);
  } else {
    return users;
  }
}

// Update a user and return new user details
var updateUser = function({id, name, age}) {
  users.map(user => {
    if (user.id === id) {
      user.name = name;
      user.age = age;
      return user;
    }
  });
  return users.filter(user => user.id === id)[0];
}

  // Root resolver
  var root = {
    user: getUser, // Resolver function to return user with specific id
    users: retrieveUsers,
    updateUser: updateUser // Include mutation function in root resolver
  };

  // Create an express server and a GraphQL endpoint
  var app = express();
  app.use('/graphql', graphqlHTTP({
    schema: schema, // Must be provided
    rootValue: root,
    graphiql: true, // Enable GraphiQL when server endpoint is accessed in browser
  }));
  app.listen(4000, () => console.log('Now browse to localhost:4000/graphql'));