Immutable vs Mutable in JS - the immutable (unable to change or mutate) values are primitive values - numbers, strings, booleans, null, undefined). While the mutable are all other objects. They are generally referred to as reference types because the object values are references to the location, in memory, that the value resides.

ImmutableVsMutable.js

// start with at string
var s = "my string";

//change its value (remember this changing of value is by value not reference)
s.toUpperCase();

// assign it to t
var t = s;

//t is now "my string"

//this is because, when we changed the value of s to be uppercase, we were actually only referring to s's value not the
// location in which it is stored. Therefore the next time it is fetched, it is the same as the originally stored value


//objects on the other hand are reference types so when we mutate them, we are mutating the value that is stored in memory
//so the next time it is use, it will pull the value from memory with the mutations included

//create and mutate
var obj = { z : 1 };
obj.y = 2;
obj.x = 3;

// obj, in memory, now stores all three properties

// a weird thing to come across is that because this is by reference, when you assign it to a new object like:
var otherObj = obj;

//otherObj is now pointing at the same location that obj is
//so changing any properties in otherObj will change them in obj, and vice versa

otherObj.a = 4;

//obj will also have a property named a with the value of 4

//if you don't want this you must create a new object and explicitly copy over the properties
var finalObj = {};

//- foreach property in obj
for(prop in obj){
  //add it to the new object
  finalObj[prop] = obj[prop];
}

great

good i learned very easily