A Brief Look at JavaScript - Web Development and Design Foundations with XHTML - page 551

Decision Making

As we've seen, we can use variables in JavaScript. We may wish to test the value of a
variable, and perform different tasks based on the variable. For instance, perhaps an
order form requires that the user enters a quantity greater than 0. We could test the
quantity input box to be sure the number entered is greater than 0. If the quantity is
not greater than 0 we could pop up an alert message instructing the user to enter a
quantity greater than 0. The if control structure looks as follows:

if (condition)
{

. . . commands to execute if condition is true
} else {
. . . commands to execute if condition is false

}

Notice that there are two types of grouping symbols used: parentheses and brackets.
The parentheses are placed around the condition and the brackets are used to encapsu-
late a block of commands. The if statement includes a block of commands to execute
if the condition is true and a block of commands to execute if the condition is false .
The brackets are aligned so that you can easily see the opening brackets and closing
brackets. It's very easy to miss a bracket when you're typing, and then have to go hunt-
ing for the missing bracket. Aligning them makes it much easier to track them visually.
As you are typing JavaScript code remember that parentheses, brackets, and quotations
always are used in pairs. If a script isn't working as intended, verify that each of these
items has a “partner.”

If the condition evaluates as true , the first command block will be executed and the
else block will be skipped. If the condition is false , the first command block will be
skipped and the else block will execute.

For the purpose of an overview, this is quite simplified, but it will give you a sense of
how conditions and the if control structure can be useful. The condition must be
something that can be evaluated as either true or false . We can think of this as a
mathematical condition. The condition will generally make use of an operator.
Table 14.4 lists commonly used comparison operators . The examples in Table 14.4
could be used as conditions in an if structure.

Table 14.4 Commonly used comparison operators

Sample Values of Quantity That
Would Result in true

Operator Description Example

= = Double equals sign (equivalent) “is

exactly equal to”

quantity = = 10 10

> Greater than quantity > 10 11, 12 (but not 10)

> = Greater than or equal to quantity > = 10 10, 11, 12

< Less than quantity < 10 8, 9 (but not 10)

< = Less than or equal to quantity < = 10 8, 9, 10

! = Not equal to quantity ! = 10 8, 9, 11 (but not 10)