2.1 Game 2302-0310 javascript  (Page 9/11)

Behavior of the function named getArea

You may recall that the area of a circle is calculated by multiplying the mathematical constant PI by the radius squared. There is no squaringoperator in JavaScript. Therefore, you can square a value by multiplying it by itself.

PI times the square of the radius

The code in the function named getArea in Listing 4 computes the area and uses the assignment operator "=" to store the result in the variable named theArea . This statement represents a case where a value is being stored in a variable(assigned to the variable) for safekeeping.

Then the code in the function extracts the value stored in the variable named theArea and returns that value. After that, the function terminates and returns control to the place in the calling script from which itwas originally called.

The variable named area

Further down the page in Listing 4 , the script declares a variable named area without using the keyword var . (Note, however, that the keyword var could have been used here. I prefer that approach for reasons that I won't get into here.)

The script calls the getArea function, passing a radius value of 3.2 as a parameter. As you learned earlier, the value returned by thefunction replaces the call to the function, which is then assigned to the variable named area .

Display the results

Then the script calls the write method to display some text followed by the value stored in the variable named area , producing the output shown in Figure 8 in the browser window.

The string concatenation operator

The code in Listing 4 exposes another operator that I will refer to as the string concatenation operator .

Note the argument list for the call to the write method in Listing 4 . In addition to being used to perform numeric addition, the plus operator "+" can be used to concatenate (join) two strings.

If two strings are joined by the + operator, the two strings will produce a new string that replaces the combination of the two originalstrings and the + operator.

If the left operand to the + operator is a string and the right operand is a numeric value (or the name of a variable containing a numeric value), thenumeric value will be replaced by a string of characters that represent that numeric value and the two strings will be concatenated into a single string.

Repetition

The last item in the list under Structured programming is repetition , and that will be the topic of this section.

Repetition (also referred to as looping) means the act of causing something to repeat.

A repetition or loop is a set of commands that executes repeatedly until a specified condition is met.

JavaScript supports three loop statements:

• while
• for
• do-while

The while loop

The while loop is not only the simplest of the three, it is also the most fundamental. It can be used to satisfy any requirement forrepetition in a JavaScript script. The other two exist solely for added convenience in some situations. Therefore, I will concentrate on the while loop, and leave the other two be discussed in a future module, if at all.