Function promisify

aws-node-graphql-api-with-dynamodb/handler.js:13–21

const dynamoDb = new AWS.DynamoDB.DocumentClient();

const promisify = foo => new Promise((resolve, reject) => {
  foo((error, result) => {
    if (error) {
      reject(error);
    } else {
      resolve(result);
    }
  });
});

const getGreeting = firstName => promisify(callback =>
  dynamoDb.get({