Object oriented design in Node.js

Question

I'm struggling with the async flow of Node.js. Assume you have class below:

function myClass() {
  var property = 'something';
  var hasConnected = false;

  this.connect = function(params) {
    //Some logic that connects to db and in callback returns if the connection is successful
    connectToSomeDB('ConnectionString', function(connectionResult) {
      hasConnected = connectionResult
    })
  };

  this.get = function(params) {
    if(hasConnected) {
      DoGetOperation() //
    }
    else {
      //Another question. What should be done here. Call the connect again?
    }
  }
}

Considering the Javascript and Node structures I certainly believe there are some major issues with my design but I cannot figure a way out since connect must be called in order for any operation to work properly. But when I do some logging after the operations:

brandNewObject = myClass();
brandNewObject.connect();
brandNewObject.get();

I observe that the get function is called before obtaining the global isConnected variable. What can I do to make this work without actually going against the async structure of Node?

Ideally, the solution I'm looking for is actually to handle 'connection' internally instead of defining a callback 'outer class'

Answer 1

There are different work-arounds for this.

One simple way is similar to what you are doing

this.get = function(params) {
    if (hasConnected) {
        DoGetOperation(params);
    } else {
        //here you can check the status of the connect. if it is still in 
        //progress do nothing. if the connection has failed for some reason
        //you can retry it. Otherwise send a response back indicating that the
        //operation is in progress.
    }
}

Another method might be to use the same async callback mechanism for your get function, which would change your method signature to something like this.

this.deferredOperations = new Array();

this.get = function(params, callback) {
    if (hasConnected) {
       //using a callback as an optional parameter makes the least 
       //impact on your current function signature. 
       //If a callback is present send the data back through it, 
       //otherwise this function simply returns the value (acts synchronously).
       if (callback !== null) {
         callback(DoGetOperation(params));
       } else {
         return DoGetOperation(params);
       }
    } else {
       this.deferredOperations.push([params,callback]);
    }
}

//connect changes now
this.connect = function(params) {
//Some logic that connects to db and in callback returns if the connection is successful
connectToSomeDB('ConnectionString', function(connectionResult) {
  hasConnected = connectionResult;
  if (hasConnected && this.deferredOperations.length > 0) {
    for (var i=0; i < this.deferredOperations.length; i++) {
      var paramFunction = this.deferredOperations.pop();
      var params = paramFunction[0];
      var func = paramFunction[1];
      DoAsyncGetOperation(params, func); //Need to write a new function that accepts a callback
    }
  }
})
};

HTH

Answer 2

You have to use callbacks.

function myClass() {
  var property = 'something';
  var hasConnected = false;

  // added second parameter
  this.connect = function(params, callback) {
    //Some logic that connects to db and in callback returns if the connection is successful
    connectToSomeDB('ConnectionString', function(connectionResult) {
      hasConnected = connectionResult;
      // Now call the callback!
      callback();
    })
  };

  this.get = function(params) {
    if(hasConnected) {
      DoGetOperation() //
    }
    else {
      //Another question. What should be done here. Call the connect again?
    }
  }
}
brandNewObject = myClass();
brandNewObject.connect({}, function () {
  // this function gets called after a connection is established
  brandNewObject.get();
});

Answer 3

Add a callback parameter to your connect method.

  this.connect = function(params, callback) {
    //Some logic that connects to db and in callback returns if the connection is successful
    connectToSomeDB('ConnectionString', function(connectionResult) {
      hasConnected = connectionResult;

      // Call the callback provided
      callback(params);
    })
  };

You can then call it like this:

brandNewObject = myClass();
brandNewObject.connect({}, function(/* optionally, "params" */) {
    brandNewObject.get();
});