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Class No. 16:

Writing Your Own JavaScript Functions

Today’s Goal

To write your own JavaScript functions.

Class Resources

Web Programming book reading for THIS class

pages 316-317 and 319-323 (skip the intervening sections for now)

Functions

Assignment Statements and Objects
Document Object Model

Web Programming book reading for NEXT class

pages 538-542

Arrays

Beginner’s Essential JavaScript Cheat Sheet

Preambles

Questions from our last class or the readings or work between classes?

User-Defined Functions

Basic Structure

the function statement
parameter list in parentheses (...)
code within curly brackets {...}
optional return statement

Important Note of Function Declaration Styles

Our book uses the following function declaration style:

source: Dean, Web Programming, p. 317

I prefer the following function declaration style:

     var DisplayHello = function() {
       let msg;
       msg = document.getElementById("message");
       msg.outerHTML = "<h1>Hello, world!</h1>";
     }

My reasons for using the var style:

the var style makes it clear that a function is actually a JavaScript variable

thus, it can be manipulated like any other variable, although you generally won’t what to change a function definition

writing the function name in CamelCase makes the name immediately distinguishable as a function name
setting the name to function makes it clear that the variable type is function
also note that I use let rather than var to declare a variable inside the function

use let for variables that will not be used outside the function
let defines local variables that are undefined outside the scope of the function

Example

 1  <head>
 2    <script type="text/javascript">
 3      // return the area of a circle given its radius
 4      var CircleArea = function( radius ) {
 5        let area = Math.PI * radius * radius ;  // compute "pi r squared"
 6        return area.toFixed( 5 ) ;              // return result to 5 decimal places
 7      }
 8    </script>
 9  </head>
10 
11  <body>
12    <table class="CircleAreas" cellspacing="0" cellpadding="5" border="1">
13      <tr>
14        <th colspan="2">Circle Areas</th>
15      </tr>
16      <tr>
17        <th>Radius</th>
18        <th>Area</th>
19      </tr>
20      <tr>
21        <td>1</td>
22        <td>
23          <script>
24            document.writeln( CircleArea( 1 ) ) ;   // call function
25          </script>
26        </td>
27      </tr>
28      <tr>
29        <td>2</td>
30        <td>
31          <script>
32            document.writeln( CircleArea( 2 ) ) ;   // call function
33          </script>
34        </td>
35      </tr>
36    </table>
37  </body>

Circle Areas

Radius Area

1 3.14159

2 12.56637

Circle Areas
Radius	Area
1	3.14159
2	12.56637

adding a little CSS...

41  <style>
42    table.CircleAreas tr td:nth-child(1) {
43      text-align : center ;
44    }
45    table.CircleAreas tr td:nth-child(2) {
46      text-align : right ;
47    }
48  </style>

Circle Areas

Radius Area

1 3.14159

2 12.56637

Run the code yourself

Circle Areas
Radius	Area
1	3.14159
2	12.56637

Parameters

definition: parameters are values passed to functions

they are used within a function to do its job
any number of parameters can be passed to a function
multiple parameters are separated by commas

no types in parameter lists

again, use Hungarian notation if you find that helpful
document each parameter so that you know what it is

you can put each parameter on a separate line if you’d like

calls aren’t even checked for the correct number of parameters
unsupplied parameters default to null , sort of ...
- if you test an unsupplied parameter against null using ==, you get true , but you can’t differentiate that from a truly null parameter
- if you test an unsupplied parameter against null using ===, you get false, which allows you to differentiate unsupplied parameters from truly null parameters
- a better test is to use typeof:
- demonstration: TestMissingParameters.html
arguments.length will tell you the number of parameters passed
declare all functions in the Web page’s <head> section
- why? what did we say that’s special about the <head> section?

The return statement

there are no declared return types

thus you can do some very sloppy things, which I certainly don’t advise

Modification of the Above Example Using a JavaScript Loop

51  <table class="CircleAreas" cellspacing="0" cellpadding="5" border="1">
52    <tr>
53      <th colspan="2">Circle Areas</th>
54    </tr>
55    <tr>
56      <th>Radius</th>
57      <th>Area</th>
58    </tr>
59    <script>
60      var radius = 1 ;
61      for ( var k = 1 ; k <= 7 ; k++ ) {
62        document.writeln( "<tr>" ) ;
63        document.writeln( "<td>" + radius + "</td>" ) ;
64        document.writeln( "<td>" + CircleArea( radius, 2 ) + "</td>" ) ;
65        document.writeln( "</tr>" ) ;
66        radius = radius * 2 ;  // could also be written: radius *= 2 ;
67      }
68    </script>
69  </table>

Circle Areas

Radius Area

1

2

4

8

16

32

64

Circle Areas
Radius	Area
1
2
4
8
16
32
64

Today’s Exercise

Goal: To write a program using a function and a table.

Procedure:

Start fresh with a brand new page for this exercise. You should still use the HTML5 template that we’ve been using throughout this course, but this will give us a fresh start.
Write a function like CircleArea in the above examples, but that computes something other than the area of a circle. For example, you might write a function that computes the area of a rectangle or a triangle. If you don’t know the formulas for those shapes, just ask.

Be sure to define your function in the <head> section of your program.

Test your function by calling it with a single line of code just to make sure that it works as you want it to.

For example, to compute the area of a rectangle, you need to pass two parameters: the width and the height of the rectangle.
You can then test your function with a single line of code something like:
document.writeln( RectangleArea( 3, 5 ) );
which should print 15 in your browser window.
Alternatively, open your browser console by typing CtrlShiftI and try:

console.log( RectangleArea( 3, 5 ) );

You are now ready to create a table to display the output of multiple calls to your function. Try using a loop like that in the last example above to generate the rows and columns of your table.
Be sure to view your page in the browser through the web server using an address in the format:

http://cssdweb.edu/username/filename.html

This is Class No. 16. It was last modified on Saturday, October 21, 2023 at 4:38 PM. Copyright © 2010-2025 by Jesse M. Heines. All rights reserved, but may be freely copied or excerpted for educational purposes with credit to the author.