Loop through AWS Lambda Nodejs SDK function

Question

I'm new to Nodejs and having trouble understand this issue: I tried to run a describe function against an array, and the AWS function seems to run after the main function has finished.

Here's the main function: (loop thru a list of ACM ARNs and check the status)

var checkCertStatus = function(resolveObj){
    var promise = new Promise(function(resolve, reject){

        console.log('1');
        var retObj='';
        resolveObj.Items.forEach(function(element) {
            var certDescribeParams = {
                CertificateArn: element.sslCertId
            };
            console.log('2');
            acm.describeCertificate(certDescribeParams, function(err, data) {
                if(err) reject(new Error(err));
                else     {
                    console.log(data.Certificate.DomainName + ': ' + data.Certificate.Status);
                    retObj+=data;
                }
            });
        });
        console.log('3');
        resolve(retObj);
        return promise;
    })
}

Based on the debug log, assuming there are 2 items need to be processed, what I got:

1
2
2
3
example.com: ISSUED
example2.com: ISSUED

Basically, I need to pass this result to the next function in the chain (with promise and stuff).

Answer 1

Welcome to Node.js! Speaking generally, it might be helpful to study up on the asynchronous programming style. In particular, you seem to be mixing Promises and callbacks , which may make this example more confusing than it needs to be. I suggest using the AWS SDK's built-in feature to convert responses to Promises.

The first thing I notice is that you are manually constructing a Promise with a resolve/reject function. This is often a red flag unless you are creating a library. Most other libraries support Promises which you can simply use and chain. (This includes AWS SDK, as mentioned above.)

The second thing I notice is that your checkCertStatus function does not return anything. It creates a Promise but does not return it at the end. Your return promise; line is actually inside the callback function used to create the Promise.

Personally, when working with Promises, I prefer to use the Bluebird library. It provides more fully-featured Promises than native, including methods such as map . Conveniently, the AWS SDK can be configured to work with an alternative Promise constructor via AWS.config.setPromisesDependency() as documented here .

To simplify your logic, you might try something along these lines (untested code):

const Promise = require('bluebird');
AWS.config.setPromisesDependency(Promise);

const checkCertStatus = (resolveObj) => {
    const items = resolveObj.Items;
    console.log(`Mapping ${items.length} item(s)`);
    return Promise.resolve(items)
        .map((item) => {
            const certDescribeParams = {
                CertificateArn: item.sslCertId,
            };
            console.log(`Calling describeCertificate for ${item.sslCertId}`);
            return acm.describeCertificate(certDescribeParams)
                .promise()
                .then((data) => {
                    console.log(`${data.Certificate.DomainName}: ${data.Certificate.Status}`);
                    return data;
                });
        });
};

We're defining checkCertStatus as a function which takes in resolveObj and returns a Promise chain starting from resolveObj.Items . (I apologize if you are not yet familiar with Arrow Functions .) The first and only step in this chain is to map the items array to a new array of Promises returned from the acm.describeCertificate method. If any one of these individual Promises fails, the top-level Promise chain will reject as well. Otherwise, the top-level Promise chain will resolve to an array of the results. (Note that I included an inessential .then step just to log the individual results, but you could remove that clause entirely.)

Hope this helps, and I apologize if I left any mistakes in the code.