- 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
- Q & A
Displaying Random Content on the Client Side
LISTING 6.1 A Random-Quote Web Page
Although this code looks kind of long, a lot of it consists of just the four quotes available for display on the page.
If you look closely at the code, you will see some lines like this:
// Create the arrays
// 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 = "Optimism is the faith that leads to achievement."; sources = "Helen Keller";
quotes = "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 = "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);
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.