Creating a Game World - Building JavaScript Games for Phones, Tablets, and Desktop - page 42

Scope of Variables
The place where you declare a variable has consequences for where you're allowed to use the
variable. Look at the variable d in the MovingSquare program. This variable is declared (and assigned
a value) in the update method. Because it's declared in the update method, you're only allowed to
use it in this method. For example, you aren't allowed to use this variable again in the draw method.
Of course, you could declare another variable called d in the draw method, but it's important to
realize that the d variable declared in update would in that case not be the same d variable declared
in the draw method.
Alternatively, if you declare a variable at the object level, you can use it anywhere, as long as you put
the name of the object in front of it. You need to use the x-position of the rectangle in both the update
and draw methods, because in the update method, you update this position, and in the draw method,
you use it to draw a rectangle at that x-position. Therefore, it's logical that this variable needs to be
declared at the object level, so that all methods belonging to the object can use the variable.

The places where a variable can be used are together called the variable's scope . In this example,
the scope of the variable d is the update method, and the scope of the variable
Game.rectanglePosition is the global scope.

What You Have Learned
In this chapter, you have learned:

How to store basic information in memory using variables



How to create objects that consist of member variables and methods



update method to change the game world through variables and
the draw method to display the game world on the screen

How to use the

