HTML and CSS Reference

In-Depth Information

Using a Conditional Operator

The code needs to run different operations based on the hours value. You can specify

these options through the use of a conditional operator. A
conditional operator
is an

operator that tests whether a specified condition is true. If the condition is true, one value

is returned; if the condition is not true, a different value is returned. The syntax of a con-

ditional operator is

(
condition
) ?
trueValue
:
falseValue

where
condition
is an expression that is either true or false,
trueValue
is the value

returned if the condition is true, and
falseValue
is the value returned if the condi-

tion is false. You can use a conditional operator to assign a value to a variable using the

statement

variable
= (
condition
) ?
trueValue
:
falseValue
;

where
variable
is the variable to which the resulting value is assigned.

Using Comparison Operators

To create expressions that have true or false values, you use comparison operators.

A
comparison operator
is an operator that compares the value of one expression to

another. One commonly used comparison operator is the less than operator (
<
), which

is used to determine whether one value is less than another. The following expression

demonstrates the use of the less than (
<
) comparison operator:

x < 100

If the value of the x variable is less than 100, then this expression is true; but if x is

greater than or equal to 100, the expression is false. Figure 11-29 lists the comparison

operators supported by JavaScript.

Figure 11-29

comparison operators

Operator

Definition

Expression

Description

==

equal to

x == y

Returns
true
if x equals y

!=

not equal to

x != y

Returns
true
if x does not equal y

>

greater than

x > y

Returns
true
if x is greater than y

<

less than

x < y

Returns
true
if x is less than y

>=

greater than or

equal to

x >= y

Returns
true
if x is greater than or equal to y

<=

less than or

equal to

x <= y

Returns
true
if x is less than or equal to y

When you want to test whether two values are equal, you use a double equal

sign (
==
) rather than a single equal sign. Thus, to test whether x is equal to 100, you use

the following expression:

The symbols in the ==

comparison operator must

be entered without a

space between the two =

symbols.

x == 100

If
x
is equal to 100, then this expression returns the Boolean value
true
; otherwise, it

returns the Boolean value
false
. Using the single equal sign (
=
) is a common program-

ming mistake; but remember that the equal sign is an assignment operator and is reserved

for setting one value equal to another, not for testing whether two values are equal.