async.race = execute in parallel, but only use the first result

[rprieto]

I found myself needing this type of function a few times:

• execute several async operations in parallel
• call done whenever the first one finished, and ignore any subsequent results

It's easily implemented as follows, but does it belong in async? Happy to submit a PR with unit tests.

async.first = function(ops, done) {
  var i = 0;
  var response = false;
  for (i = 0; i < ops.length; ++i) {
    ops[i](function(err, data) {
      if (!response) {
        response = true;
        done(err, data);
      }
    });
  }
};

[fritx]

async.any ?

[rprieto]

The way I understand async.any is is runs the same asynchronous predicate on multiple items, and calls the callback with true or false (depending if any items matched).

In my case I'd like to run several (different) async functions, and return the result of the first one to finish.
For example:

// we have potential services that can return the same data
// call both in parallel and return whichever comes back first
async.first([
  getDataFromServiceX,
  getDataFromServiceY
], done);

or

// call done() when the long operation finishes
// or after 1 sec, whichever happens first
async.first([
  function(next) { longRunningOperation(next); }
  function(next) { setTimeout(next, 1000);     }
], done);

[fritx]

Yeah, I was wrong.. Haha

[rprieto]

No worries :)

[vaiwa]

I would like this feature too =)

Now I hacked async.parralel so that it returns data in err and err in data, but it is really ugly and confusing.

[kumavis]

+1 do want

[kumavis]

an alternate mode would be to ignore errors, and keep waiting for the first non-failure response, until all have been attempted

[ORESoftware]

+1

[ORESoftware]

@rprieto you might need to create some logic to check the result of the request. for example, if the request returns an error, we might not want the first one, or if the first request doesn't have the data you want, you might not want the first result either. So there might need to be some conditional check in there somewhere.

So for generic usage, we want not just the first result but the first result that meets some condition. So your version of async.first might want to take one more argument in the form of a function that contains some conditional logic that returns a boolean.

[ORESoftware]

something that looks like:

async.first = function(ops, func, done) {  // developer defined func to check condition
  var length = ops.length;
  var responses = 0;
  var success = false;
  for (var i = 0; i < length; i++) {
    ops[i](function(err, data) {
      responses++;
      if (!success) {
        if(func(err,data)){  
          success = true;
          done(err, data);
          }
          else if(responses >= length){  //we have received all responses, but none met the condition
          success = true;   // probably should rename success var, but ok for now
          done(new Error('all responses received, but no responses met condition'), data);
          }
         else{
           //do nothing, wait for the next response
         }
      }
    });
  }
};