How to correctly perform async I/O in map iterator in node.js? [duplicate]

Question

Possible Duplicate:
Cannot use “map” function within async module

I've got a problem like this:

    var paths = ['path1', 'path2', 'path3', ...]; //Some arbitrary array of paths

    var results; //I need to collect an array of results collected from these paths

results = paths.map(function(path){
  var tempResult;

  GetData(path, function(data){ //Third-party async I/O function which reads data from path
    tempResult = data;
  });

  return tempResult;
});

console.log(results); //returns something like [nothing, nothing, nothing, ...]

I can imagine why it happens (return tempResult fires before the async function returned any data - it's slow after all), but can't quite see how to make it right.

My guess is async.map might help, but I fail to see how right away.

Maybe someone more experienced in asynchronous programming might explain the way?

Actually, async.map's only example covers a case that is almost identical to yours. — lanzz
– lanzz, Commented Oct 21, 2012 at 11:58
Hmm... Probably I don't get something... what should the signature for iterator be? Should it not return value (like result = GetData(...))? — Garo Robe
– Garo Robe, Commented Oct 21, 2012 at 12:19
Nevermind. Found a bit more helpful example in this thread . Thanks for the answer. — Garo Robe
– Garo Robe, Commented Oct 21, 2012 at 12:23

Kaustubh Karkare · Accepted Answer · 2012-10-21 12:46:09Z

2

You could try something like:

async.map(paths,function(path,callback){
    GetData(path,function(data){ callback(null,data); });
},function(error,results){
    if(error){ console.log('Error!'); return; }
    console.log(results);
    // do stuff with results
});

As you can see, you'll need to shift the code that processes the results into the function to be passed into async.map.

answered Oct 21, 2012 at 12:46

Kaustubh Karkare

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1 Comment

Garo Robe Over a year ago

That's perfect! It explains how to get result from asynchronous iterator so much better than async docs. Thank you.

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