How to use Amazon DynamoDB JavaScript SDK with promise

Question

I am working with Amazon DynamoDB and Express.

I need to render a view based on the data from a Amazon DynamoDB table.

My code works fine when I work with callback API instead of trying to use promise.

But I want to use promise because to keep my code clean, otherwise I need to call res.send() from inside the callback of the docClient.scan(params).

My code for working with promise as follows I can't figure out what is wrong;

async function test(params){
    AWS.config.loadFromPath('./awsconfigtest.json');
    let docClient = new AWS.DynamoDB.DocumentClient();
    await docClient.scan(params).promise();
}

Below is the content of the route file;

/* GET home page. */
router.get('/', function(req, res, next) {

    let scanResults ;
    let params = {
        TableName: 'dummy'
    };

    test(params).then((data,err)=>{
        console.log(data,err);
        data.Items.forEach(function (element, index, array) {
            scanResults.push({name: element.name, nodeId: element.nodeId});
            console.log(element.name + " (" + typeof element.nodeId + ")");
        });
    });

    console.log(scanResults);
    res.render("index",{nodes:scanResults});    
});

hoangdv · Accepted Answer · 2019-10-25 03:04:21Z

Main reason is your test function does not get back anything - You forgot return keyword.

But, I think the best practice is don't mix async/await syntax with Promise solve syntax (.then .catch).

This is way use async/await:

async function test(params){
    AWS.config.loadFromPath('./awsconfigtest.json');
    let docClient = new AWS.DynamoDB.DocumentClient();
    return await docClient.scan(params).promise(); // !!! You forgot return keyword
}

/* GET home page. */
router.get('/', async function (req, res, next) { // async function
  try {
    let scanResults;
    let params = {
      TableName: 'dummy'
    };

    const data = await test(params); // await for `data`
    console.log(data);

    data.Items.forEach(function (element, index, array) {
      scanResults.push({ name: element.name, nodeId: element.nodeId });
      console.log(element.name + " (" + typeof element.nodeId + ")");
    });

    console.log(scanResults);
    res.render("index", { nodes: scanResults });

  } catch (err) {
    res.status(500).send(err); // handle exception
  }
});

Matthew Weeks · Accepted Answer · 2019-10-24 17:57:12Z

0

I believe you could use promisify from node util.

Something like this:

const util = require('util');

async function test(params){
    AWS.config.loadFromPath('./awsconfigtest.json');
    let docClient = new AWS.DynamoDB.DocumentClient();
    const scan = util.promisify(docClient.scan);
    await scan(params);
}

The promisify function converts a function that takes a callback as its last value, and converts it into a promise, that will reject the err value, and resolve the data value (from something like docClient.scan(params, (err, data) => {}))

answered Oct 24, 2019 at 17:57

Matthew Weeks

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Comments

H. N. Mishra · Accepted Answer · 2023-06-06 04:49:58Z

0

 try {
    const params = {TableName : NOTES_TABLE_NAME};
    const notes = await util.promisify((cb) => documentClient.scan(params, cb))();
    cb(null, send(200, notes));
  }
  catch (err){
    cb(null, send(500, err.message));
  }

Worked for me.

edited Jun 6, 2023 at 4:49

answered Jun 6, 2023 at 4:27

H. N. Mishra

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