Variables

A variable is basically a "container" in which you can store information. If you compared it to a cup for a second, a cup could contain any amount of any type of liquid, or it could contain no liquid at all. The value stored within this variable, or container however, can be changed during a script. You can refer to a variable by name in order to either see its value or if you want to change it. A given script can have an unlimited number of variables as long as each has a unique name.

Variable Naming Rules

• Variable names are case sensitive, so Coke, CoKe and coke are three different variables. This becomes important to get in the habit of standardizing how you name your variables.
• Variable names must start with an underscore or a letter, so coke and _coke and coke0 are all valid variable names where as 0coke or 123_coke are not valid variable names. Remembering these rules will save you a lot of trouble in scripting.
• When naming your variables, you have to remember that you can NOT use math symbols. These include things like , " ' . () ; etc.
• There are reserved words in JavaScript, words that are used in the language, reserved words that you cannot use the name your variables, the list is very long and you're welcome to look it up but a few examples are (new, do, else, if, import, this, switch)

Types of Variables

• Integer Numbers
  Integer variables can hold the basic counting numbers, either positive or negative.  These values can be operated on using arithmetic operators such as addition, subtraction, multiplication, and division.
  Example: 4, 564, -34
   
• Floating-Point Numbers
  Floating-point variables can hold fractional numbers that include a decimal point.  These variables can also be used with the standard arithmetic operators.
  Example: 4.5, 9.876, -0.93
   
• String Variables
  String variables are simply words and sentences.  Always use quotation marks with string variables.
  Example: "hello world", "this is a variable"
   
• Boolean Variables
  Boolean variables are a unique variable type.  They can only hold the values true and false.  These variables are most used to test the condition of a statement.  These variables can also hold the values 0 and 1, for false and true, respectively.
  Example: True, False, 1, 0

Declaring a Variable

You create a variable by using the var statement, and then afterwards listing the name of the variable you want to use, using the naming conventions listed above. It is also possible to declare a variable without the var statement but it is a good habit to use it. See Figure 2.1 for an example of declaring the variable email and setting its value to zexipher@yahoo.com.

Lifetime of Variables

Each variable that you declare within a function can only be accessed from within that function. After the function is left the variable no longer exists. It is only recreated after you next access the function, but with the default value. Not any changes that you may have made to the variable during prior uses. These variables (variables you declared within a function) are called local variables. You can actually use the same name for two local variables if they are used in separate functions because each variable is only recognized by the function in which it was declared.

If you declare a variable outside of a function on your page, it is called a global variable. This means that the lifetime of the variable starts when you open the page and ends when the page is closed.

Code Documentation

While in very simple scripts it is very easy to keep track of all of your variables, especially if given relevant names. Using code documentation can sometimes work to your advantage. This means using comments to explain each variable you declare within your function. For examples of how to use comments in JavaScript see Figure 2.2 below. Only a very simple explanation is needed but is useful, especially if you intend to share your work with others. See Figure 2.3 for an example of code documentation. This is not required in JavaScript but simply a useful tool for future users of your scripts.