Recursive Async Looping in NodeJS

Question

I'm trying to do a recursive async loop to trace all the children of a particular object from a third-party lib in nodejs.

Heres the pseudo code:

var tracer = function(nodes){
  var promises [];

  nodes.forEach(function(node){

    // trace returns a promise ...
    var promise = builder.trace(node)
    promises.push(promise);

    promise.then(function(tree){

      // if we had children, get those
      if(tree.children.length){
        promises.push.apply(promises, tracer(tree.children));
      }
    });   

  });

  return promises;
};

RSVP.all(tracer(myArr)).then(function(allTrees){ ... });

but I can't put my finger on how to get them all to resolve correctly and returns the results in one array.

How does tracer(myArr) even have .then? In this code, you're not returning a promise to use .then on. — Marko Gresak
– Marko Gresak, Commented Jan 15, 2015 at 23:03

Benjamin Gruenbaum · Accepted Answer · 2015-01-16 09:12:24Z

4

You must not push the recursive promises on the array in the delayed callback. Instead, you'll need to push a promise that represents the recursive results (resolves with those delayed produced promises) right away. Luckily, you even get exactly that back from that then call.

Additionally, I would swap out the each for a map, and do RSVP.all immediately inside the function, for not expecting the caller to deal with that.

function tracer(nodes){
  var promises = nodes.map(function(node){
    // trace returns a promise ...
    var promise = builder.trace(node)
    var recusivePromise = promise.then(function(tree){
      // if we had children, get those
      if (tree.children.length)
        return tracer(tree.children));
      else
        return node;// the leaf node itself
    });
    return recusivePromise; // which will resolve with the `tracer(…)` result
                            // or the leaf
  });
  return RSVP.all(promises);
}

tracer(myArr).then(function(allTrees){ … });

edited Jan 16, 2015 at 9:12

Benjamin Gruenbaum

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answered Jan 15, 2015 at 23:52

Bergi

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3 Comments

amcdnl Over a year ago

Note: This return a lists of lists vs a flat array.

Bergi Over a year ago

@amcdnl: Yes, it's supposed to match the tree structure. If you want a flat array, make each recursivePromise resolve with an array (by returning leafs in singleton arrays), and add a concat step after .all().

Bergi Over a year ago

@DanKanze: It's basic functional programming :-)

amcdnl · Accepted Answer · 2015-01-16 00:19:21Z

0

I ended up going with a counter type approach ...

var traceDeps = function(parents, cb){
  var count = 0, 
    trees = [], 
    trace = function(nodes){
      nodes.forEach(function(node){
        count++;
        builder.trace(node).then(function(tree){
          trees.push(tree);

          if(tree.children.length){
            trace(tree.children);
          }

          count--;
          if (count === 0) cb(trees);
        });
      });
    };

  trace(parents);
};

traceDeps(myArr, function(trees){ ... });

answered Jan 16, 2015 at 0:19

amcdnl

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4 Comments

Bergi Over a year ago

Why so complicated? No need to use callbacks and counters when you can use promises. Also you are ignoring errors with this approach.

Bergi Over a year ago

If any of the trace() promises is getting rejected, you won't notice (and instead wait indefinitely for your callback that is never called)

Benjamin Gruenbaum Over a year ago

This solution is inferior to Bergi's and ignores errors. You should consider using his instead.

amcdnl Over a year ago

Ya, I agree. Thanks for the post @Bergi

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