javascript

Chapter 8: The Browser Object Model
In this example, the variable num is incremented every half second until it finally reaches the maximum
number, at which point the interval is canceled. This pattern can also be implemented using timeouts, as
shown here:
var num = 0;
var max = 10;
function incrementNumber() {
num++;
}
//if the max has not been reached, set another timeout
if (num < max) {
setTimeout(incrementNumber, 500);
} else {
alert(“Done”);
}
setTimeout(incrementNumber, 500);
Note that when you’re using timeouts, it is unnecessary to track the timeout ID, because the execution
will stop on its own and continue only if another timeout is set. This pattern is considered a best practice
for setting intervals without actually using intervals. True intervals are rarely used in production
environments because it ’ s possible that one interval will begin before the previous one has finished
executing. Using timeouts, as in the preceding example, ensures that can ’ t happen. Generally speaking,
it ’ s best to avoid intervals.
System Dialogs
The browser is capable of invoking system dialogs to display to the user through the alert() ,
confirm() , and input() methods. These dialogs are not related to the web page being displayed in the
browser and do not contain HTML. Their appearance is determined by operating - system and/or
browser settings rather than CSS. Additionally, each of these dialogs is synchronous and modal, meaning
code execution stops when a dialog is displayed, and resumes after it has been dismissed.
The alert() method has been used throughout this book. It simply accepts a string to display to the
user. When alert() is called, a system message box displays the specified text to the user, followed by a
single OK button. For example, alert( “ Hello world! “ ) renders the dialog box shown in Figure 8 - 3
when used with IE on Windows XP.
Figure 8-3
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