Data Types and Variables
Computer programs are processors of data. They take numbers, letters, and other
special characters and manipulate them in some fashion for the purpose of
producing information. They attempt, in other words, to assign meaning to
bits and pieces of data by applying actions such as calculating, categorizing,
arranging, presenting, or otherwise manipulating and organizing the data to
types: numeric, string, and Boolean.
Numeric Data Type
recognizes two kinds of numbers: integer positive or negative whole numbers composed
only of the decimal digits 0 - 9; floating-point numbers are composed of the decimal
digits plus a decimal point. Thus, the value 12345 is an integer number and the value
in decimal notation (as above), in scientific notation (123.45E6), in hexadecimal
(0XFF00CC), or in octal (012345). Numeric data are defined for the purpose of
applying mathematical operations to them.
String Data Type
String data are composed of the lower-case alphabetic characters a - z, the
upper-case characters A - Z, the numerals 0 - 9, plus special
characters such as $, #, &, _, and others. Strings are identified by
enclosing them in quotes: "This is a string of characters." Character strings
are treated as single blocks of text that can be put together (concatenated) to create
a longer character string from the separate strings, or they can be taken apart to
extract substrings composed of portions of the string.
Since quotes are used to surround and define text strings, a special situation arises
when you want to include quotation marks as part of a text string. You cannot just
include them in the middle of the string as follows:
"Here is a string with a "quote" marks inside."
pairs of quote marks surround and define a string, this line is interpreted as three
separate elements: the string "Here is a string with a "
is followed by the word quote,
which is followed by a second string " marks inside."
This is clearly not what is intended.
There are a couple of ways to handle this situation. You can mark embedded quotes with a preceding
backslash character (\), technically called escaping the quote character:
"Here is a string with \"quote\" marks."
This format indicates that the embedded quotation mark does not enclose a string; it
is a character within a string. You can also enclose the entire string inside
single quotes (apostrophes) to differentiate between the two quote types:
'Here is a string with "quote" marks.'
This latter technique of alternating quote characters is probably the easiest
way to distinguish between embedded quotes and enclosing quotes, although it does
beg the question of what to do about quoted strings that contain both embedded
quotes and embedded apostrophes:
'Here's a string with "quote" marks.'
as ending the string begun with the initial single quote. So, you need to
"Here's a string with \"quote\" marks." or 'Here\'s a string with "quote" marks.'
The point is, you need to be careful when composing text strings that themselves enclose
quote marks and apostrophes to make sure that the entire string is treated as a single entity.
following useful escape sequences:
Boolean Data Type
Boolean data consists of two values: true and
false. This data type
is used in script decision making, a topic covered later where you will
see the application of this data type.
Null Value Data Type
An empty value. Assigning a null value to a variable indicates that the
variable contains no usable value.
A variable that has never had a value assigned to it, has not been declared, or does not exist.
contain multiple values as opposed to a single or scalar value. Three composite
types will be discussed in more detail later.
determine the data type of a variable. This is useful because the data type of
syntax of the operator is shown below:
The typeof operator can return the following values:
|Return Value||Returned For
|Boolean||True or False
|Number||Integers and Floating-point numbers
|Object||Arrays and null variables
A computer program that processes data must set aside storage areas in computer memory where
numeric, string, and Boolean data can be placed, and from which they can be accessed in order to
process them. Programs create these storage areas by defining program variables.
A variable is a named memory location. Once defined, it is available as a place to store
data temporarily while a program, or script, is running. The script can assign data to that
location, retrieve data from the location for processing, and put processing results back into
must be followed. Variable names
- must begin with an alphabetic character or the underscore ( _ ) character
- can be composed of letters, numerals, or the underscore character
- cannot contain blank spaces
Otherwise, you are free to name your variables as you wish. It makes sense, of course, to
assign names that are meaningful and that help identify the kinds of data being stored in the
associated memory locations.
lower-case characters. Therefore, variable myName references a
different memory location from my_name or MY_NAME.
Make sure you pay careful attention to the case of the variable names you assign and use them
statement with the keyword var. This word is followed by the name
you wish to assign to the variable as shown in the general format in Figure 2-1.
This format is illustrated in the following examples of declaring variables inside a script.
Here, three different storage locations are named and set aside to contain data; plus, the
variable names suggest the kinds of data to be placed in these locations. Any time a script needs
to refer to or access the stored data, it does so through the names assigned to these locations.
This is an important point. A script reference to a variable name is a reference to the actual data
value residing in the named memory location.
Notice also in the above example that a semicolon is used to terminate each declaration
statement. Doing so permits more than one statement to be placed on a single line of code by
explicitly terminating each statement. If, however, only a single statement appears on a line,
use of the semicolon is optional. The convention followed in these tutorials is to always
Assigning Data to Variables
formats for the assignment statement include those shown in Figure 2-2.
variable = number;
variable = "string"
variable = true|false
Figure 2-2. General formats to assign data to variables.
The data value appearing on the right of the equal sign is
assigned to (placed in) the variable named on the left of the equal sign. This value can be an
integer or floating-point number, it can be a text string enclosed in quotes, or it can be one
of the Boolean values true or false. For example,
if you have declared the following variables,
then you can place data into the these variables with the assignments:
myNumber = 12345;
myString = "A text string";
myTest = true;
As you can see, the keyword var is used only the first time a
variable is declared. Subsequently, the variable name by itself references the existing variable.
You can see the results of these statements below. Click on the radio buttons in sequence
to execute the statements and watch the declarations and assignments of values to the three variables.
Figure 2-3. Declaring and assigning values to variables.
Incidentally, you can combine the two actions of declaring a variable and assigning data to it
within a single statement:
var myNumber = 12345;
var myString = "A text String ";
var myTest = true;
This combined declaration and assignment is appropriate to establish the initial value for
a variable when the value is known in advance. In many cases, however, you will not know the value
In this case, you need only to declare the variable without assigning its value.
Technically, you do not have to use the var keyword. You can
declare a variable and assign a value with, simply,
myNumber = 12345
myString = "A Text String";
myTest = true;
Still, it is conventional to follow the formal method of declaring a variable with the keyword
var. You will also notice in the examples that blank spaces appear
before and after the equal sign in the assignment statements. You can use spaces freely in
In each of the examples above, the process of assigning data to a variable does not require
of the variable based on the value assigned to the variable. Unlike most programming
The data type is always determined by the type of data assigned to the variable.
For this reason, the data type of variable can change depending on the data assigned
to the variable. This is in contrast to other languages such as Java, Visual Basic,
and C++ that are strongly typed languages where you are required to explicitly
declare the data type of a variable.
Assigning Variables to Variables
In the above examples, literal data values are assigned to variables—an actual number,
text string, or Boolean value is placed in the variable. You also can assign the contents of one
variable to another variable with the syntax shown in Figure 2-4.
variable1 = variable2;
Figure 2-4. General format to assign a variable value to a different variable.
Consider the following five lines of code:
firstString = "Hello World";
secondString = firstString;
firstString = "Goodbye World!";
In the first two lines of code, two variables are declared, firstString
and secondString. At this point, two memory areas have been set
aside for storing data. The contents of these two variables are null, meaning that their data
values are undefined.
In the third line, the text string "Hello World" is assigned to
variable firstString. In the next line, the value stored in
firstString is assigned to variable secondString.
That is, the data value "Hello World" that is sitting in variable
firstString is copied into variable secondString.
At this point, both variables contain the text string "Hello World".
In the last line of code, the text string "Goodbye World" is assigned to
variable firstString. The data value "Hello
World" is replaced by the value "Goodbye World".
Two important points: (1) When the contents of a variable is assigned to a different variable,
it is copied from the source into the destination variable; it is not moved from the
source to the destination variable leaving the source variable blank, or null. (2) When a data
value is assigned to a variable, it replaces the current contents of the variable
with the new value. These operations are illustrated below.
Figure 2-5. Assigning variables to variables.