CODEWITHSUNDEEP
javascriptnode.jsgeneratorq
An SO User
An SO User

Reputation: 24998

How to use yield with my own functions?

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?

Upvotes: 0

Views: 481

Answers (2)

dsdenes
dsdenes

Reputation: 1035

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);
  });

Upvotes: 1

Bergi
Bergi

Reputation: 664297

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.

Upvotes: 1

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