Solution: Creating a Basic GraphQL Endpoint

// Diffrent imports
import { Application, Router } from 'https://deno.land/x/oak@v6.0.1/mod.ts';
import { applyGraphQL, gql } from "https://deno.land/x/oak_graphql@0.6.2/mod.ts";

// Configure Deno application
const app = new Application();

app.use(async (ctx, next) => {
    const start = Date.now();
    await next();
    const ms = Date.now() - start;
    ctx.response.headers.set("X-Response-Time", `${ms}ms`);
});

// Types definition
const types = gql`
    type Car {
        id: String
        brand: String
        registration: String
        price: Float
        description: String
    }

    input CarInput {
        brand: String
        registration: String
        price: Float
        description: String
    }

    type CarRental {
        id: String
        car_id: String
        user_id: String
        total_price: Float
        start_date: String
        end_date: String
    }

    input CarRentalInput {
        car_id: String
        user_id: String
        total_price: Float
        start_date: String
        end_date: String
    }

    type User {
        id: String
        firstName: String
        lastName: String
    }

    input UserInput {
        firstName: String
        lastName: String
    }

    type ResolveType {
        done: Boolean
    }
    
    type Query {
        getAllCars: [Car]
        getAllCarRentals: [CarRental]
        getCar(id: String): Car
        getCarRental(id: String): CarRental
    }

    type Mutation {
        addCar(input: CarInput!): ResolveType!
        addCarRental(input: CarRentalInput!): ResolveType!
        addUser(input: UserInput!): ResolveType!
    }
`;

// Some dummy data
const cars = [
    {
        id: "0",
        brand: "Toyota",
        registration: "XT-564-FS",
        price: 15.3,
        description: "A Car for renting"
    }
];
const carRentals = [
    {
        id: "0",
        car_id: "0",
        user_id: "0",
        total_price: 153,
        start_date: "01/01/2022",
        end_date:"11/01/2022"
    }
];
const users = [
    {
        id: "0",
        firstName: "Deno",
        lastName: "Tutorial"
    }
];

// Resolvers definition
const resolvers = {
    Query: {
        getAllCars: () => {
            return cars;
        },
        getAllCarRentals: () => {
            return carRentals
        },
        getCar: (_parent: any, { id }: { id: string }) => {
            console.log(`getCar() id=(${id})`);
            return cars.find((car) => car.id === id);
        },
        getCarRental: (_parent: any, { id }: { id: string }) => {
            console.log(`getCarRental() id=(${id})`);
            return carRentals.find((carRental) => carRental.id === id);
        }
    },
    Mutation: {
        addCar: (_parent: any, { input: { brand, registration, price, description } }: any, _context: any, _info: any) => {
            console.log(`addCar() brand=(${brand}), registration=(${registration}), price=(${price}) description=(${description})`);
            cars.push(
                {
                    id: ((cars.length)++).toString(),
                    brand,
                    registration,
                    price,
                    description                }
            );
            return {
                done: true,
            };
        },
        addCarRental: (_parent: any, { input: { car_id, user_id, total_price, start_date, end_date } }: any, _context: any, _info: any) => {
            console.log(`addCarRental() car_id=(${car_id}), user_id=(${user_id}), total_price=(${total_price}), start_date=(${start_date}), end_date=(${end_date})`);
            carRentals.push(
                {
                    id: ((carRentals.length)++).toString(),
                    car_id,
                    user_id,
                    total_price,
                    start_date,
                    end_date
                }
            );
            return {
                done: true,
            };
        },
        addUser: (_parent: any, { input: { firstName, lastName } }: any, _context: any, _info: any) => {
            console.log(`addUser() firstName=(${firstName}) and lastName=(${lastName})`);
            users.push(
                {
                    id: ((users.length)++).toString(),
                    firstName,
                    lastName
                }
            );
            return {
                done: true,
            };
        }
    }
};

// Add the GraphQL service
const GraphQLService = await applyGraphQL<Router>({
    Router,
    typeDefs: types,
    resolvers: resolvers
})

// Start application
app.use(GraphQLService.routes(), GraphQLService.allowedMethods());

await app.listen({ port: 8000, secure: true, certFile: "/usercode/server.cert", keyFile: "private.key" });