In the following code, what is 'this' referring to.?

$(x).on('click', y);

sets y as click handler for x. The actual JavaScript code is

document.querySelector( "button" ).onclick = function() {

...// "this" is used in here

};

Since onclick is a member of the button object, "this" refers to the button.

This piece of code executes upon complete page load. It sets on click function for the button.

Once you click the button it would set element,probably div with id vacation, to text 'From 399' and then remove the button so it can't be clicked again

In Java, "this" is used to refer to whatever object currently has the "focus" in the program. In the code that you gave, it would be the button that was selected in the line before.

Often when you are in a subroutine you have no way of knowing what object will be selected. "This" means uses what the current object is. You have to backtrack in the code to determine what "this" is.