2.1 Game 2302-0310 javascript  (Page 5/11)

The function definition

The function definition in Listing 1 consists of the three lines of code following the comment that begins with "//Define the function..."

As explained earlier, this function definition contains:

• The function keyword
• The function name: getHalf
• An argument list containing a single parameter named incomingParameter
• A function body enclosed in a pair of matching curly brackets

Simpler than the surface area of a sphere

The function named getHalf is somewhat simpler than one that could be used to calculate the surface area of a sphere, but the basic conceptis the same. This function expects to receive one parameter.

The code in the body of the function uses the division operator, "/", to divide the value of the incoming parameter by 2. Then it returns the result ofthat calculation. When the function returns, the return value will replace the call to the function in the calling script.

Two calls to the function

The function is called twice in the body of the script in Listing 2 , passing a different value for the parameter during each call.

Each call to the function named getHalf is embedded as one of the elements in theargument list following document.write .

write is also a function

Although I didn't mention this earlier because you weren't ready for it yet, write is also the name of a function. However, the write function is predefined in JavaScript and we can use it to display information in the browser window without having to defineit first.

(Actually, write is a special kind of a function that we call a method , but you don't need to worry about that unless you want to dig much deeper into theobject-oriented aspects of JavaScript.)

Two calls to the getHalf function

The script in Listing 2 calls the function named getHalf twice in two different locations, each of which is embedded in the argument list of a call to the write method. Each call passes a different parameter value to the getHalf function.

The order of operations

When a call to a function or method (such as the call to the write method) includes a call to another function or method in its argument list, thecall to the function in the argument list must be completed before the call can be made to the function having the argument list. Therefore in each of these twocases, the call to the getHalf function must be completed before the call to the write method can be executed.

Output from script in Listing 2

Each call to the getHalf function returns a value that is half the value of its incoming parameter.

As I mentioned earlier, when the function returns, the returned value replaces the call to the function. Therefore, ineach case, the returned value from the call to the getHalf function becomes part of the argument list for the write method before that method is called. This causes the two calls to the write method in Listing 2 to display the values returned from the calls to the getHalf function as shown in Figure 2 .