HTML and CSS Reference
In-Depth Information
the programmer chooses the name. Heres an example of a function definition that returns the product of
the two arguments. As the name indicates, you could use it to compute the area of a rectangle.
function areaOfRectangle(wd,ln) {
return wd * ln;
}
Notice the
return
keyword. This tells JavaScript to send the result of the function back to us. In our
example, this lets us write something like
rect1 = areaOfRectangle(5,10)
, which would assign a value
of 50 (5
10) to our
rect1
variable. The function definition would be written as code within the
script
element. It might or might not make sense to define this function in real life because it is pretty easy to
write multiplication in the code, but it does serve as a useful example of a programmer-defined function.
Once this definition is executed, which probably would be when the HTML file is loaded, other code can
use the function just by calling its name, as in
areaOfRectangle(100,200)
or
areaOfRectangle(x2-
x1,y2-y1)
.
The second expression assumes that
x1
,
x2
,
y1
,
y2
refer to coordinate values that are defined elsewhere.
Functions also can be called by setting certain tag attributes. For example, the
body
tag can include a
setting for the
onLoad
attribute:
<body onLoad="init();">
My JavaScript code contains the definition of a function I call
init
. Putting this into the
body
element
means that JavaScript will invoke my
init
function when the browser first loads the HTML document or
whenever the player clicks on the reload/refresh button. Similarly, making use of one of the new features
of HTML5, I could include the button element:
<button onClick="throwdice();">Throw dice </button>
This creates a button holding the text
Throw dice
. When the player clicks it, JavaScript invokes the
throwdice
function I defined in the
script
element.
The
form
element, to be described later, could invoke a function in a similar way.
Conditional statements:
if
and
switch
The craps game has a set of rules. One way to summarize the rules is to say, if it is a first-throw situation,
we check for certain values of the dice throw. If its not the first throw, we check for other values of the
dice throw. JavaScript provides the
if
and
switch
statements for such purposes.
The
if
statement is based on
conditions,
which
can be a comparison or a check for equality—for
example, is a variable named
temp
greater than 85 or does the variable named
course
hold the value
"Programming Games". Comparisons produce two possible logical values—true or false. So far youve
seen values that are numbers and values that are strings of characters. Logical values are yet another
data type. They are also called
Boolean
values, after the mathematician, George Boole. The condition and
check that I mentioned would be written in code as
temp>85
and
course == "Programming Games"
Read the first expression as: Is the current value of the variable
temp
greater than 85?