What is the scope of javascript callback and anonymous functions?

Question

I have written the following code using node.js and riak-js. I have a recursive function walk that should be a list of JSON documents, but instead returns an empty list... why? how to fix?

require('riak-js');

var walk = function(bucket, key, list){ 
  if(list == undefined){
    var list = new Array();
  } 
  db.get(bucket, key)(function(doc, meta){     
     list.push(doc);
     if(meta.links.length > 0 && meta.links[0].tag == 'child'){
       walk(bucket, meta.links[0].key, list);
     }   
  });
  return list; 
}

familytree = walk('smith', 'walter', []);

Thanks in advance!

Based on the guide, I think your syntax is off. It should be db.get(bucket, key, function(doc, meta)... — Matthew Flaschen
– Matthew Flaschen, Commented Oct 1, 2010 at 4:43
@Matthew Flaschen is right. Your syntax for db.get is wrong. — Shripad Krishna
– Shripad Krishna, Commented Oct 1, 2010 at 6:09
actually my syntax is correct for the version of riak-js I'm using (v0.2.2) - if you look at the guide, you'll see that it says "Note: This guide is only applicable to riak-js 0.3.0" — roder
– roder, Commented Oct 1, 2010 at 12:51

Daniel Vassallo · Accepted Answer · 2010-10-01 04:13:23Z

3

You get an empty array because db.get() is asynchronous. It returns immediately without waiting for the callback to be invoked. Therefore when the interpretor reaches the return list statement, list is still an empty array.

It is a fundamental concept in Node.js (and even in browser scripting) that everything is asynchronous (non-blocking).

answered Oct 1, 2010 at 4:13

Daniel Vassallo

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