Node.js synchronously loop or iterate over asynchronous statements

With recursion the code is pretty clean. Wait for the http response to come back then fire off next attempt. This will work in all versions of node.

var urls = ['http://stackoverflow.com/', 'http://security.stackexchange.com/', 'http://unix.stackexchange.com/'];var processItems = function(x){  if( x < urls.length ) {    http.get(urls[x], function(res) {      // add some code here to process the response      processItems(x+1);    });  }};processItems(0);

A solution using promises would also work well, and is more terse. For example, if you have a version of get that returns a promise and Node v7.6+, you could write an async/await function like this example, which uses some new JS features.

const urls = ['http://stackoverflow.com/', 'http://security.stackexchange.com/', 'http://unix.stackexchange.com/'];async function processItems(urls){  for(const url of urls) {    const response = await promisifiedHttpGet(url);        // add some code here to process the response.  }};processItems(urls);

Note: both of these examples skip over error handling, but you should probably have that in a production app.

To loop and synchronously chain asynchronous actions, the cleanest solution is probably to use a promise library (promises are being introduced in ES6, this is the way to go).

Using Bluebird, this could be

Var p = Promise.resolve();forEach(sets, function(item, index, arr) {    p.then(new Promise(function(resolve, reject) {         http.get(theUrl, function(res) {         ....         res.on('end', function() {              ...              resolve();         }    }));});p.then(function(){   // all tasks launched in the loop are finished});

I found out that I wasn't releasing my mysql connections after I was done with each call and this tied up the connections causing it to fail and appear to be an issue with synchronization.

After explicitly calling connection.release(); it caused my code to work 100% correctly even in an asynchronous fashion.

Thanks for those who posted to this question.