How to use node.js generators with DynamoDB

Question

I am trying to use generators to put and get data from my local dynamoDB. My code, so far, returns quite large objects. But I'm not quite sure if I actually interact with the database. And if so, I can't figure out how to actually retrieve data. Another thing is the callback function which is needed when not using generators. Am I supposed to leave it out? If not, how would I yield the result to the next()-function?

Any help is deeply appreciated! :-)

So far my code looks like this:

'use strict';

var AWS = require('aws-sdk');

AWS.config.update({accessKeyId: 'akid', secretAccessKey: 'secret'});
AWS.config.update({region: 'us-west-1'});
AWS.config.apiVersion = '2015-10-01';
//Using DynamoDB Local
var dyn = new AWS.DynamoDB({ endpoint: new AWS.Endpoint('http://localhost:8000') });

//Wrap the async calls in a generator functions
function* putItemGenerator(putParams) {
    yield dyn.putItem(putParams);
}
function* getItemGenerator(getParams) {
    yield dyn.getItem(getParams);
}

class User {
    //The constructor creates a new user in the 
    //database and inserts his ID and name
    constructor (args) {
        this.userId = args.userId;
        this.name = args.name;

        let putParams = {
            "TableName": "Users",
            "Item": {
                userId: { S: this.userId },
                name: { S: this.name }
            }
        };

        //Greate a generator and run it.
        let result = putItemGenerator(dyn, putParams).next();

        console.log(" === PUT RESULT === ");
        console.log(result.value);
    }

    //Get the User from the Database
    getUser() {
        var getParams = {
            "TableName": "Users",
            "ConsistentRead": true,
            "Key": {
                "userId": { S: this.userId },
            }
        };

        let result = getItemGenerator(dyn, getParams).next();

        console.log(" === GET RESULT === ");
        console.log(result.value);

        return result.value;
    }
}

var user = new User( {
    userId: "1337",
    name: "John Doe"
} );

user.getUser();

/*
//I created the table with this script.

'use strict';

var AWS = require('aws-sdk');

var createTables = function() {
    AWS.config.update({accessKeyId: 'akid', secretAccessKey: 'secret'});
    AWS.config.update({region: 'us-west-1'});
    AWS.config.apiVersion = '2015-10-01';
    let dyn = new AWS.DynamoDB({ endpoint: new AWS.Endpoint('http://localhost:8000') });

    params = {
        TableName : "Users",
        KeySchema: [
            { AttributeName: "userId", KeyType: "HASH" }
        ],
        AttributeDefinitions: [  
            { AttributeName: "userId", AttributeType: "S" }
        ],
        ProvisionedThroughput: {       
            ReadCapacityUnits: 1, 
            WriteCapacityUnits: 1
        }
    };
    dyn.createTable(params, function(err, data) {
        if (err)
            console.log(JSON.stringify(err, null, 2));
        else
            console.log(JSON.stringify(data, null, 2));
    });
}

createTables();
*/

Answer 1

I found out that what I want to do is not possible. A getter that retrieves it's value by an I/O operation can not return a value. A best practice would be to return a so called promise. This can be used by the calling function as soon as the value is available. Generators can make the code look more beautiful and easy to read.