How to use yield with my own functions?

Question

I am new to the Generator concept. My understanding is that if a function returns a Promise, then it can be used with yield. So I have a very small node.js script that looks like this:

Q.fcall(function*(){
  var url = "mongodb://" + config.host + ":" + config.port + "/" + config.db;
  var db = yield MongoClient.connect( url );
  var data = yield makeRequest();
  console.log( data );
  db.close();
});


function makeRequest(){
  var deferred = Q.defer();
  request({
      "method" : "GET",
      "url" : "....",
      "headers" : {
          "Accept" : "application/json",
          "user_key" : "...."
      }
  },function(err,data){
      if( err ){
          deferred.reject( err );
      }else{
          deferred.resolve( data );
      }
  });
  return deferred.promise;
}

I know this works because I am porting it from the callback hell style to generator style. However, I do not see the data in console.log.

What do I need to change to make this work?

*"My understanding is that if a function returns a Promise, then it can be used with `yield`."* Generators (and `yield`) have nothing to do with promises. They can be used together to implement something like `async/await`, but `yield` itself doesn't treat promises differently from any other value. — Felix Kling, Mar 07 '16 at 07:25
This should answer your question: [What happens when promise is yielded in javascript?](http://stackoverflow.com/q/33947850/218196) — Felix Kling, Mar 07 '16 at 07:27
@FelixKling thanks ! I was reading this http://stackoverflow.com/questions/17516952/trying-to-understand-generators-yield-in-node-js-what-executes-the-asynchron?rq=1 when you posted that :) — An SO User, Mar 07 '16 at 07:29
@FelixKling I did a little digging around and changed `Q.fcall( )` to `co()` and it worked. So what does `co` exactly do? :) — An SO User, Mar 07 '16 at 07:33
@LittleChild as far as I understand `co` it basically keeps calling `passedInGenerator.next()` until the `{done: true}` object is returned or an error occurs. — jmunsch, Mar 07 '16 at 08:02
Also there is a decent and simple example here that shows the idea: http://stackoverflow.com/questions/21846390/getting-a-promises-value-via-yield-co — jmunsch, Mar 07 '16 at 08:15

score 1 · Accepted Answer · answered Mar 07 '16 at 09:23

Q.fcall is not suited to work with generator functions. You need to use the dedicated generator methods for that, in your case Q.spawn:

Q.spawn(function*() {
  var url = "mongodb://" + config.host + ":" + config.port + "/" + config.db;
  var db = yield MongoClient.connect( url );
  try {
    var data = yield makeRequest();
    console.log( data );
  } finally {
    db.close();
  }
});

Without a runner, the generator won't be automatically advanced asynchronously.

score 1 · Answer 2 · answered Mar 21 '16 at 16:14

Consider using co that can wrap generators and make the callable. It returns promise.

const co = require('co');
const promisedGenerator = co.wrap(function* () {
  yield whatever1();
  yield whatever2();
  return databaseRequest(); 
});

promisedGenerator()
  .then(databaseResult => {
    console.log(databaseResult);
  });

How to use yield with my own functions?

2 Answers2