- Refresher on the Different Types of Scripting
- Displaying Random Content on the Client Side
- Understanding the Document Object Model
- Using window Objects
- Working with the document Object
- Accessing Browser History
- Working with the location Object
- More About the DOM Structure
- Working with DOM Nodes
- Creating Positionable Elements (Layers)
- Hiding and Showing Objects
- Modifying Text Within a Page
- Adding Text to a Page
- Changing Images Based on User Interaction
- Thinking Ahead to Developing HTML5 Applications
- Summary
- Q & A
- Workshop
This chapter is from the book
This chapter is from the book
This chapter is from the book
Displaying Random Content on the Client Side
In Chapter 4, you learned the basics of JavaScript, such as how it fits into a web page. As an example of doing something dynamic on the client side, this section walks you through adding random content to a web page through JavaScript. You can use JavaScript to display something different each time a page loads. Maybe you have a collection of text or images you find interesting enough to include in your pages.
I’m a sucker for a good quote. If you’re like me, or plenty of other people creating personal websites, you might find it fun to incorporate an ever-changing quote into your web pages. To create a page with a quote that changes each time the page loads, you must first gather all your quotes, along with their respective sources. You then place these quotes into a JavaScript array, which is a special type of variable in programming languages that is handy for holding lists of items.
After the quotes are loaded into an array, the JavaScript that’s used to pluck out a quote at random is relatively small. Listing 6.1 contains the complete HTML and JavaScript code for a web page that displays a random quote each time it loads.
LISTING 6.1 A Random-Quote Web Page
<!DOCTYPE html>
<html lang="en">
<head>
<title>Quotable Quotes</title>
<script type="text/javascript">
<!-- Hide the script from old browsers
function getQuote() {
// Create the arrays
var quotes = new Array(4);
var sources = new Array(4);
// Initialize the arrays with quotes
quotes[0] = "Optimism is the faith that leads to achievement.";
sources[0] = "Helen Keller";
quotes[1] = "If you don't like the road you're walking, " +
"start paving another one.";
sources[1] = "Dolly Parton";
quotes[2] = "The most difficult thing is the decision to act, " +
"the rest is merely tenacity.";
sources[2] = "Amelia Earhart";
quotes[3] = "What's another word for thesaurus?";
sources[3] = "Steven Wright";
// Get a random index into the arrays
i = Math.floor(Math.random() * quotes.length);
// Write out the quote as HTML
document.write("<p style='background-color: #ffb6c1;
text-align:center'>\"");
document.write(quotes[i] + "\"");
document.write("<em>- " + sources[i] + "</em>");
document.write("</p>");
}
// Stop hiding the script -->
</script>
</head>
<body>
<h1>Quotable Quotes</h1>
<p>Following is a random quotable quote. To see a new quote just
reload this page.</p>
<script type="text/javascript">
<!-- Hide the script from old browsers
getQuote();
// Stop hiding the script -->
</script>
</body>
</html>
Although this code looks kind of long, a lot of it consists of just the four quotes available for display on the page.
The large number of lines between the first set of <script></script> tags creates a JavaScript function called getQuote(). After a function is defined, it can be called in other places in the same page, which you see later in the code. Note that if the function existed in an external file, the function could be called from all your pages.
If you look closely at the code, you will see some lines like this:
// Create the arrays
and
// Initialize the arrays with quotes
These are code comments. A developer uses these types of comments to leave notes in the code so that anyone reading it has an idea of what the code is doing in that particular place. After the first comment about creating the arrays, you can see that two arrays—initialized using the keyword var—are created—one called quotes and one called sources, each containing four elements:
var quotes = new Array(4); var sources = new Array(4);
After the second comment (about initializing the arrays with quotes), four items are added to the arrays. Let’s look closely at one of them, the first quote by Helen Keller:
quotes[0] = "Optimism is the faith that leads to achievement."; sources[0] = "Helen Keller";
You already know that the arrays are named quotes and sources. But the variables to which values are assigned (in this instance) are called quotes[0] and sources[0]. Because quotes and sources are arrays, each item in the array has its own position. When you’re using arrays, the first item in the array is not in slot #1—it is in slot #0. In other words, you begin counting at 0 instead of 1, which is typical in programming and is true in JavaScript as well as PHP—just file that away as an interesting and useful note for the future (or as a good trivia answer). Therefore, the text of the first quote (a value) is assigned to quotes[0] (a variable). Similarly, the text of the first source is assigned to source[0].
Text strings are enclosed in quotation marks. However, in JavaScript, a line break indicates an end of a statement, so the third quote would cause problems in the code if it were written like this:
quotes[2] = "The most difficult thing is the decision to act, the rest is merely tenacity.";
Therefore, you see that the string is built as a series of strings enclosed in quotation marks, with a plus sign (+) connecting the strings (this plus sign is called a concatenation operator):
quotes[2] = "The most difficult thing is the decision to act, " + "the rest is merely tenacity.";
The next chunk of code definitely looks the most like programming; this line generates a random number and assigns that value to a variable called i:
i = Math.floor(Math.random() * quotes.length);
But you can’t just pick any random number—the purpose of the random number is to determine which of the quotes and sources should be printed, and there are only four quotes. So this line of JavaScript does the following:
Uses Math.random() to get a random number between 0 and 1. For example, 0.5482749 might be a result of Math.random().
Multiplies the random number by the length of the quotes array, which is currently 4; the length of the array is the number of elements in the array. If the random number is 0.5482749 (as shown previously), multiplying that by 4 results in 2.1930996.
Uses Math.floor() to round the result down to the nearest whole number. In other words, 2.1930996 turns into 2; remember that we start counting elements in an array at 0, so rounding up would always mean a chance that we would refer to an element that does not exist.
Assigns the variable i a value of 2 (for example).
The rest of the function should look familiar, with a few exceptions. First, as you learned in Chapter 4, document.write() is used to write HTML that the browser then renders. Next, the strings are separated to clearly indicate when something needs to be handled differently, such as escaping the quotation marks with a backslash when they should be printed literally (\) or when the value of a variable is substituted. The actual quote and source that are printed are the ones that match quotes[i] and sources[i], where i is the number determined by the mathematical functions noted previously.
But the act of simply writing the function doesn’t mean that any output will be created. Further on in the HTML, you can see getQuote(); between the two <script></script> tags—that is how the function is called. Wherever that function call is made, that is where the output of the function will be placed. In this example, the output displays below a paragraph that introduces the quotation.
Figure 6.1 shows the Quotable Quotes page as it appears when loaded in a web browser. When the page reloads, there is a one-in-four chance that a different quote displays—it is random, after all!
)
FIGURE 6.1 The Quotable Quotes page displays a random quote each time it is loaded.
Keep in mind that you can easily modify this page to include your own quotes or other text that you want to display randomly. You can also increase the number of quotes available for display by adding more entries in the quotes and sources arrays in the code. And of course, you can modify the HTML output and style it however you’d like.
If you use the Quotable Quotes page as a starting point, you can easily alter the script and create your own interesting variation on the idea. If you make mistakes along the way, so be it. The trick to getting past mistakes in script code is to be patient and carefully analyze the code you’ve entered. You can always remove code to simplify a script until you get it working, and then add new code one piece at a time to make sure each piece works.