I don't understand how this code works in javascript

Question

//classical JS does not provide a set data type--use JS object to build prime set:   
    var Set = function() {
        Set.prototype.add = function(o) {
        this[o] = true; 
      }
    }

    //Big-Time Important: 
    var s = new Set();
        s.add(2);
        s.add(3);
        s.add(5);
        s.add(7);
        s.add(11);
        s.add(13);
        s.add(17);
        s.add(19);
        s.add(23);   

This code works with a game in which it adds in a bunch of prime numbers and you must guess which one is the correct one. However the code: var Set = function () then goes into another function I don't understand the process of what is happening or how is that working with the set object created below.

Answer 1

this is object oriented java script style.. the prototype gives access to add variables and/or methods to the object in this case Set object. You can think of like below. Set is a class, you can create object by new Set()

function Set(){
 this.add = function(o){
   this[o]= true;
 }
} 

Answer 2

Before ES6 Javascript did not have built in in sets. So in this code you are creating a set.

var Set = function() {
        Set.prototype.add = function(o) {
        this[o] = true; 
      }
    }

The code above is javascript way of creating a "class".

Set.prototype.add = function(o) {
            this[o] = true; 
          }

The lines of code above is Javascript way of adding method called "add" to Set Class.

this[o] = true;

The line above is adding a property o and setting it's value to true. It is same as this.o = true