GraphQL course implementing a resolver function

Now is time to provide the implementation that specifies exactly how the server return the “greeting value“.

Note: last GraphQL chapter was GraphQL course defining a schema

We are going to do that by creating an object called “resolvers” the resolver need to match to the type definition

const resolvers = {}

The the object need a property called Query & Query will be a nested object because it represent a type

const resolvers = {
  Query: {}
}

And Query object have a property called “greeting” just like the field that we declared in the type definition and this will be a function that will return the value our of greeting.

const resolvers = {
  Query: {
    greeting: () => 'Hello world!'
  }
}

So, that function will be called by the GraphQL engine every-time that the client sends the greeting query or in other words this function will be called to resolve the value of the “greeting field” this is why is called a resolver function and also why we call the hole of object as “resolvers“.

So, resolve have Query and greeting as same as type definition:

If you see the greeting type definition is string type so the resolver function is returning an string:

The next step is create our server, so to do that we need to import the ApolloServer class from “apollo-server”:

const { ApolloServer, gql } = require("apollo-server");

ApolloServer class is needed because we need a new instance from that Class like this:

const server = new ApolloServer({});

Then the ApolloServer constructor take an object where we can pass configuration properties:

const server = new ApolloServer({ typeDefs, resolvers });

Finally we need to start our server calling .listen() method and this method also take a configuration object as parameters:

server
  .listen({ port: 9000 })

Since .listen() method return a promise over <ServerInfo> we can call .then() that take ServerInfo as argument and log a message to the console.

const server = new ApolloServer({ typeDefs, resolvers });
server
  .listen({ port: 9000 })
  .then((serverInfo) => console.log(`Server running at ${serverInfo.url}`));

Now is time to run our GraphQl sever by running “node server.js” in the terminal:

All server.js code:

const { ApolloServer, gql } = require("apollo-server");

const typeDefs = gql`
  type Query {
    greeting: String
  }
`;

const resolvers = {
  Query: {
    greeting: () => "Hello World!",
  },
};

const server = new ApolloServer({ typeDefs, resolvers });
server
  .listen({ port: 9000 })
  .then((serverInfo) => console.log(`Server running at ${serverInfo.url}`));

By Cristina Rojas.