Asynchronous code to synchronous built-in

So, not so long ago, I had to face the rather popular task of converting asynchronous code to synchronous as part of a loop. In my case, it was working with the AmazonAPI productSearch methods. Everything would be fine, but this API just doesn’t like it when it is accessed too often, and I needed to interrogate the status of products in a cycle.

In this article, I will use a practical example to talk about the implementation of the method that I used to solve my problem. Let's get started.

For implementation we will need: promises (I will use the q library), nodejs version with generator support, experimental asynchronous function (we will use setTimeout).

var q = require("q");
function* itemGenerator(data)
{
  var i, len = data.length;
  for(i = 0; i < len; i++)
  {
    yield data[i];
  }
}
function oldIterator(data)
{
  var i = -1, len = data.length;
  return {
    "next": function()
    {
      i++;
      return {
        "done": i == len,
        "value": data[i]
      };
    }
  }
}
function main()
{
  var def = q.defer(), items = [1, 2, 3, 4, 5];
  (function foo(gen)
  {
    var genItem = gen.next(), item = genItem.value;
    if(genItem.done)
    {
      def.resolve(true);
      return;
    }
    console.log("start call for", item);
    setTimeout(function()
    {
      console.log("end call for", item);
      foo(gen);
    });
  })(itemGenerator(items))
  return def.promise;
}
main().then(function(flag)
{
  console.log("promise has been resolved", flag);
});

The result of this script will be:

> node async_sync.js
start call for 1
end call for 1
start call for 2
end call for 2
start call for 3
end call for 3
start call for 4
end call for 4
start call for 5
end call for 5
promise has been resolved true

As we see, all of our asynchronous calls go in synchronous mode. To support older versions (not supporting generators), I added the “oldIterator” function, which will work similarly to a generator.

PS : this code will work similarly in JavaScript, it’s enough to replace the “Q” library with the native “Promise”.

That's all, thanks for watching!