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CD-31Chapter 5 .Scripts and HTML Documents dialog boxes turned off, error indications are displayed subtly at the left edge of the browser window s status bar. An alert icon and message ( Error on page. ) appear in the status bar. If you double-click the icon, the error dialog box appears (see Figure 5-1). Be sure to expand the dialog box by clicking the Show Details button. Unless you turn on script error dialog boxes and keep them coming, you have to train yourself to monitor the status bar when a page loads and after each script runs. Figure 5-1: The expanded IE error dialog box For NN 4.07 and later, the status bar is also your first indication of a script error. A message appears in the status bar that instructs you to go to the location javascript: to see the error details. Viewing the details of the error requires different steps, depending on the Navigator version. For NN 4.07 and all subsequent 4.x versions, choose File . Open and enter javascript: For NN6, choose Tasks . Tools . JavaScript Console. The JavaScript console window (a separate window from the Java console) opens to reveal the error message details (see Figure 5-2). You can keep this window open all the time if you like. Unless you clear the window, subsequent error messages are appended to the bottom of the window. Understanding error messages and doing something about them is a very large subject, reserved for advanced discussion in Chapter 45. During this tutorial, however, you can use the error messages to see if you have perhaps mistyped a script from a listing in the book.
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CD-30 Part II . JavaScript Tutorial Listing 5-8: Running a Script from User Action Here is some body text.

Not every object must have an event handler defined for it in the HTML, as shown in Listing 5-8 only the ones for which scripting is needed. No script state ments execute in Listing 5-8 until the user clicks the button. The alertUser() function is defined as the page loads, and it waits to run as long as the page remains loaded in the browser. If it is never called upon to run, there s no harm done. The last scenario for when script statements run also involves functions. In this case, a function is called upon to run by another script statement. Before you see how that works, it helps to read through the next lesson (Chapter 6). Therefore, I will hold off on this example until later in the tutorial. Viewing Script Errors In the early days of JavaScript in browsers, script errors displayed themselves in very obvious dialog boxes. These boxes were certainly helpful for scripters who wanted to debug their scripts. However, if a bug got through to a page served up to a non-technical user, the error alert dialog boxes were not only disruptive, but also scary. To prevent such dialog boxes from disturbing unsuspecting users, the browser makers tried to diminish the visual impact of errors in the browser win dow. Unfortunately for scripters, it is often easy to overlook the fact that your script contains an error because the error is not so obvious. Recent versions of IE and NN have different ways of letting scripters see the errors. In IE5+, you can set its preferences so that scripts do not generate error dialog boxes (got to Tools . Internet Options . Advanced . Browsing and find the checkbox entry that says Display a notification about every script error ). Even with error
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CD-29Chapter 5 .Scripts and HTML Documents introduce you briefly to a concept covered in more depth in Chapter 7: the function. A function defines a block of script statements summoned to run some time after those statements load into the browser. Functions are clearly visible inside a Here is some body text. Don t worry about the curly braces or other oddities in Listing 5-7 that cause you concern at this point. Focus instead on the structure of the document and the flow. The entire page loads without running any script statements, although the page loads the done() function in memory so that it is ready to run at a moment s notice. After the document loads, the browser fires the onLoadevent handler, which causes the done()function to run. Then the user sees the alert dialog box. Getting a script to execute in response to a user action is very similar to the preceding example for running a deferred script right after the document loads. Commonly, a script function is defined in the Head portion, and an event handler in, say, a form element calls upon that function to run. Listing 5-8 includes a script that runs when a user clicks a button.
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CD-28 Part II . JavaScript Tutorial comment. A pair of forward slashes (no space between them) is the most common way to include a comment in a script. You add comments to a script for your benefit. They usually explain in plain language what a statement or group of statements does. The purpose of including comments is to remind you six months from now how your script works. When Script Statements Execute Now that you know where scripts go in a document, it s time to look at when they run. Depending on what you need a script to do, you have four choices for determining when a script runs: . While a document loads . Immediately after a document loads . In response to user action . When called upon by other script statements The determining factor is how the script statements are positioned in a document. While a document loads immediate execution Your first script in Chapter 3 (reproduced in Listing 5-6) runs while the document loads into the browser. For this application, it is essential that a script inspects some properties of the navigator object and includes those property values in the content being rendered for the page as it loads. It makes sense, therefore, to include the Deferred scripts The other three ways that script statements run are grouped together as what I called deferred scripts. To demonstrate these deferred script situations, I must
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CD-27Chapter 5 .Scripts and HTML Documents saw at the end of Chapter 3 in an illustration of your first script running on an old browser, the results can be disastrous to a page. You can reduce the risk of old browsers displaying the script lines by playing a trick. The trick is to enclose the script lines between HTML comment symbols, as shown in Listing 5-5. Most nonscriptable browsers completely ignore the content between the comment tags, whereas scriptable browsers ignore those comment symbols when they appear inside a The odd construction right before the ending script tag needs a brief explana tion. The two forward slashes are a JavaScript comment symbol. This symbol is necessary because JavaScript otherwise tries to interpret the components of the ending HTML symbol (–>). Therefore, the forward slashes tell JavaScript to skip the line entirely; a nonscriptable browser simply treats those slash characters as part of the entire HTML comment to be ignored. Despite the fact that this technique is often called hiding scripts, it does not disguise the scripts entirely. All client-side JavaScript scripts are part of the HTML document and download to the browser just like all other HTML. Furthermore, you can view them as part of the document s source code. Do not be fooled into think ing that you can hide your scripts entirely from prying eyes. JavaScript Statements Virtually every line of code that sits between a tag pair is a JavaScript statement. To be compatible with habits of experienced program mers, JavaScript accepts a semicolon at the end of every statement. Fortunately for newcomers, this semicolon is optional. The carriage return at the end of a state ment suffices for JavaScript to know the statement has ended. A statement must be in the script for a purpose. Therefore, every statement does something relevant to the script. The kinds of things that statements do are . Define or initialize a variable . Assign a value to a property or variable . Change the value of a property or variable . Invoke an object s method . Invoke a function routine . Make a decision If you don t yet know what all of these mean, don t worry you will by the end of this tutorial. The point I want to stress is that each statement contributes to the scripts you write. The only statement that doesn t perform any explicit action is the
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CD-26 Part II . JavaScript Tutorial Listing 5-3: Scripts in the Head and Body You also are not limited to one Handling older browsers Only browsers that include JavaScript in them know to interpret the lines of code between the tag pair as script statements and not HTML text for display in the browser. This means that a pre-JavaScript browser not only ignores the tags, but it also treats the JavaScript code as page content. As you
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CD-25Chapter 5 .Scripts and HTML Documents Listing 5-1: Scripts in the Head On the other hand, if you need a script to run as the page loads so that the script generates content in the page, the script goes in the portion of the document, as shown in Listing 5-2. If you check the code listing for your first script in Chapter 3, you see that the script tags are in the Body because the script needs to fetch information about the browser and write the results to the page as the page loads. Listing 5-2: A Script in the Body It s also good to know that you can place an unlimited number of If you forget the closing script tag, the script may not run properly and the HTML elsewhere in the page may look strange. Although you don t work with it in this tutorial, another attribute works with more recent browsers to blend the contents of an external script file into the current document. An SRCattribute (similar to the SRC attribute of an tag) points to the file containing the script code. Such files must end with a .js extension. The tag set looks like the following: All script lines are in the external file, so no script lines are included between the start and end script tags in the document. Tag positions Where do these tags go within a document? The answer is, anywhere they re needed in the document. Sometimes it makes sense to include the tags nested within the …tag set; other times it is essential that you drop the script into a very specific location in the … section. In the following four listings, I demonstrate with the help of a skeletal HTML document some of the possibilities of tag set. This is common usage in HTML where start and end tags encapsulate content controlled by that tag, whether the tag set is for a form or a paragraph. Depending on the browser, the