Extract data from variables inside multiple callbacks in javascript

Question

Extract data from variables inside multiple callbacks in javascript

I'm ok with javascript and callbacks, but I'm getting really annoyed at this and need to call on the the world of stackoverflow for help.

I have written a function, to be used in the following way:

var meth = lib.funcTion(a,b); // meth should hold an array of properties { c, d } once executed

So now inside lib.js, we have a structure like:

exports.funcTion = function (a,b) {
    database.connect(params, function(err,get){
          get.query(querylang, function(err, results){
                 var varsIwantToReturn = { var1: results[i].foo, var2: results[i].bar };
          });
    });
// Now how do i return 'varsIwantToReturn'?
};

I have seen some things about incorporating callback() into the function, but I'm not exactly sure how this works. I've also seen some people use exec() - again, im not sure on how or why to use it.

Please help :) thanks in advance.

javascript
node.js
callback

Answer 1

Well, it's all asynchronous so if you attempt to return it - it'll return undefined. In JavaScript (Sans the new yield keyword) functions execute from top to bottom synchronously. When you make an IO call like a database call - it still executes synchronously. In fact- when varIwantToReturn gets population the function has long run and terminated.

What is left is to do the same thing async functions like database.connect and get.query do and have the function take a callback:

exports.function = function (a,b, callback) {
    database.connect(params, function(err,get){
          if(err) return callback(err, null); // don't suppress errors
          get.query(querylang, function(err, results){
               if(err) return callback(err, null); // don't suppress errors
               var varsIwantToReturn = { var1: results[i].foo, var2: results[i].bar };
               callback(null, varsIwantToReturn);
         });
    });
};

Then you'd call it like

myExportedFunction(myA,myB, function(err, resp){
    if(err) recoverFromError(err);
    // varsIWantToReturn are contained in `resp`
});