Vfrdtyky

This is a method from Twitter package from npm.

  T.get('search/tweets', params, function(err, data, response) {

  if(!err){

    // This is where the magic will happen

  } else {

    console.log(err);

  }

})

I know that normally you'd write a method with callback like this

    T.prototype.get = function(url, options, callback) {

    //code here

    callback();



}

My question is how are the "err", "data", and "response" values are passed to callback function. Is it by doing the following?

callback(error, returnedTweets, reponseObject);

Yes, the parameters are sent to the callback by the caller of the callback which in your example would be the internal implementation of T.get(). So, if you get three parameters on the callback like this function(err, data, response) ..., then it's because that's how the callback was called with three arguments that are of the appropriate type to match that:

T.prototype.get = function(url, options, callback) {

    //code here

    callback(err, data, response);



}

It does not matter what the arguments that are passed are named locally. So, internal to the T.get() implementation, those arguments may have different names:

T.prototype.get = function(url, options, callback) {

    //code here

    callback(error, returnedTweets, reponseObject);

}

It is the values of those variables that are passed to the callback. The name they have inside the implementation is meaningless to the callback itself.

The name you give them when you declare the callback in this code is also irrelevant:

T.get('search/tweets', params, function(err, data, response) {

    // callback implementation here

});

You can pick any names for the callback arguments you want. The callback arguments are passed in a specific order so whatever you name the first argument in your callback declaration is what will have the value of the first argument that was passed when the callback was called. Arguments in Javascript are by order/position, not by name. For example, you could have named them like this:

T.get('search/tweets', params, function(arg1, arg2, arg3) {

    // callback implementation here

});

And, the code would still work properly, though it would be a lot less readable because one would have to just know that arg1 is the error value and arg2 is the data and so on. So, most developers pick meaningful names that help describe what is expected in that argument. But, the name you pick for a callback argument is entirely up to you.