How are input parameters filled in javascript method chains?

Question

I am trying to really understand the details of how javascript works. During method chaining, sometimes one method returns to another method that has a named input parameter

Answer 1

If I understand correctly

what does this data refer to? How is it filled?

You mean how it works? It depends on how the callback gets called. For example:

function Lib() {}

Lib.prototype.text = function(callback) {
  var data = 'hello world';
  callback(data); // `data` is the first parameter
  return this; // chain
};

var lib = new Lib();

lib.text(function(data){
  console.log(data); //=> "hello world"
});