Organizing Game Objects - Building JavaScript Games for Phones, Tablets, and Desktop - page 152

and now the following instructions:

var someNumber = 10;
square(someNumber);

After executing these instructions, the value of someNumber still is 10 (and not 100). Why is this?
Because when the square function is called, the number parameter is passed by value . The variable
f is a local variable in the method that initially contains the value of the someNumber variable. In the
method, the local variable f is changed to contain f * f , but this doesn't change the someNumber
variable, because it's another location in memory. Because non-primitive objects are passed by
reference , the following example will result in a changed value of the object passed as a parameter:

function square(obj) {
obj.f = obj.f * obj.f;
}

var myObject = { f : 10 };
square(myObject);
// myObject.f now contains the value 100.

Whenever a JavaScript script is running, there are a number of references and values in memory.
For example, if you look at Figures 11-2 and 11-3 , you see that the Ball objects contain both values
and references to other objects (such as Vector2 objects or Image objects).

Null and Undefined
Whenever you declare a variable in JavaScript, initially its value is set to undefined :

var someVariable;
console.log(someVariable); // will print 'undefined'.

In JavaScript, you can also indicate that a variable is defined but currently doesn't refer to any
object. This is done by using the null keyword:

var anotherCannon = null;

Because you haven't yet created an object (using the new keyword), the memory looks like what is
depicted in Figure 11-10 .

anotherCannon null

Figure 11-10. A variable pointing to null

So, indicating that a variable isn't yet pointing to anything is done by assigning null to it. It's even
possible to check in a JavaScript program whether a variable is pointing to an object or not, like this:

if (anotherCannon === null)
anotherCannon = new Cannon();