Perform a set of recursive HTTP GET calls, and wait for them all

Question

I have a REST service, offering a list of 'Json' objects, and each object may potentially have a link for another resource of its own class. Starting with a particular one, I need to fetch them all, performing a recursive http call. So I wrote:

var steps = [];
var recursiveLookup = function(processId) {
  return $.ajax({
    url: SERVER_URL + processId,
    success: function (activity) {
      // Activity has a set of json objects called steps
      var rtn = activity.path.map(step => {
        if (step.type != "Node") {
          steps.push(step);
        } else {
          return recursiveLookup(step.subProcessIntanceId);
        }
      }).filter(a => a != undefined);
      return rtn;
    }
  });
}

That would correctly load all objects into the global steps var. I need to be sure the method has finished, so I wrote:

var promises = recursiveLookup(processId);
Promise.all(promises).then(function () {
   console.log(steps);
});

But it's not working, as the 'recursiveLookup' is returning the promise of $.ajax, instead of the set of promises pretended to be returned with the success method.

Furthermore, is it possible to get the steps as a returned value from the 'recursiveLookup' method instead, of using it as a global variable?

Possible duplicate of [How do I return the response from an asynchronous call?](https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — Peter B, Dec 29 '17 at 12:45
is it me or your function has 2 returns? I mean you first return rtn, then try to return the ajax itself. — Cr1xus, Dec 29 '17 at 13:02
"Furthermore, is it possible to get the steps as a returned value from the 'recursiveLookup' method instead, of using it as a global variable?" You can use success: $.proxy(function(activity) { #code }, steps) — Cr1xus, Dec 29 '17 at 13:10
Can't your server produce the entire recursive tree response in one request? — James, Dec 29 '17 at 13:42
Could, but there's a scenario where I don't need the recursive calls, and performance it's not a problem (it's hosted on the same LAN) — El pocho la pantera, Dec 29 '17 at 13:54

HMR · Accepted Answer · 2017-12-29T13:37:53.050

Nested recursion is not within my confort zone but maybe this will work:

var recursiveLookup = function(processId,steps=[]) {
  return $.ajax({
    url: SERVER_URL + processId,
  })
  .then(
    function (activity) {
      // Activity has a set of json objects called steps
      steps = steps.concat(
        activity.path.filter(
          step => step.type !== "Node"
        )
      );
      return Promise.all(
        activity.path.filter(
          step => step.type === "Node"
        )
        .map(
          step=>
            recursiveLookup(step.subProcessIntanceId,steps)
        )
      ).then(
        result=>steps.concat(result)
      )
    }
  );
}

For tail call optimization to work the last thing the function does should be to call the recursive function but I think in promise chains it doesn't matter too much.

Seems good but concat returns a new array, so I should do steps = steps.concat(...), and doing that doesn't alter the array received on parameters... — El pocho la pantera, Dec 29 '17 at 13:27
activity.path.filter( step => step.type !== "Node" ).map( s=> steps.push(s) ); — El pocho la pantera, Dec 29 '17 at 13:31

Negi Rox · Answer 2 · 2017-12-29T13:13:32.653

-2

You should not use the success parameter if you want to work with promises. Instead, you want to return a promise, and you want to use then to transform the results of a promise into something different, possibly even another promise.

function request(page) {    
…
// return the AJAX promise
return $.ajax({
    url: '/echo/json/',
    method: 'POST',
    dataType: 'json',
    data: {
        delay: 1,
        json: JSON.stringify(ret)
    }
 });
}

function requestOddsFrom(page, items) {
return request(page).then(function(data){
    if (data.currentPage > data.totalPage) {
        return items;
    } else {
        var filtered = data.items.filter(function(el){ return el%2 == 1; });
        return requestOddsFrom(data.currentPage + 1, items.concat(filtered));
    }
 });
}

function requestAll(){
return requestOddsFrom(1, []);
}

 requestAll().then(function(items) {
   console.dir(items);
});

for more info jQuery Recursive AJAX Call Promise

How do I return the response from an asynchronous call?

edited Dec 29 '17 at 13:13

answered Dec 29 '17 at 12:56

Negi Rox

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That is absurd. `async:false` is a terrible practice and is deprecated. Using it will cause warnings in dev tools console not to ever use it – charlietfl Dec 29 '17 at 12:59
its depends on your need dude. not every language is perfect you need to modify it according to your need – Negi Rox Dec 29 '17 at 13:04
Well using deprecated approaches is not the way to do it. Especially in a recursive call! – charlietfl Dec 29 '17 at 13:06
Sorry Negi, that's realy the way it is. Maybe in Node you can get away with it but synchronous web requests in browser will hang everything up. You can check [here](https://stackoverflow.com/a/47678417/1641941) for information about why ES uses callback or promises for asynchronous operations (or operations that do not return a value immediately like network request) – HMR Dec 29 '17 at 13:10

Perform a set of recursive HTTP GET calls, and wait for them all

2 Answers2