HTML and CSS Reference
In the next section, you'll learn how to work with comparison operators to compare
one value with another.
Problem Solving: eval() or not eval() ?
The eval() method is a quick and easy way to evaluate text strings of numeric expressions
misused than the eval() method. The general syntax of the eval() method is
eval( string )
code can be a mathematical expression as discussed earlier, but it also can be a collection
eval(“x = 5; document.write(x)”);
As a result of running this statement, a browser would write the value 5 to the Web docu-
ment. You can make the text string itself a variable, as in the following statements that
use eval() to create a document.write() command that writes the value 10 to the Web
var y = 10;
eval(“document.write(“ + y + “)”);
code. That code then can be altered in response to events initiated within a browser or by
a user. You'll often find legacy pages using eval() extensively to perform these types of
programming tricks. However, using eval() for this purpose is greatly discouraged for the
• The code runs more slowly and uses up resources because there is no opportunity for the
browser to compile or cache the code contained in the eval() method.
• Storing code within the eval() method is a security breach, opening your program to
malicious code that can be inserted from outside agents.
• Code within the eval() method is not available to debugging tools in case of a
Almost anything that can be done using eval() also can be done with properly writ-
ten code, avoiding the security breaches and other problems associated with eval() . In
general, you should use eval() only for well-defined tasks such as evaluating an expression
that can be evaluated in no other way. Several sites on the Web provide valuable work-
arounds to avoid the misuse of the eval() method.
Working with Conditional, Comparison, and
Hector wants the countdown clock to display the current time using 12-hour format
rather than 24-hour format. In 24-hour format, the hour values range from 0 hours (repre-
senting 12:00 a.m.) up to 23 hours (representing 11:00 p.m.). The time 14:35 in 24-hour
format is equivalent to 2:35 p.m. in 12-hour format. To convert 24-hour time to 12-hour
time, the code needs to apply the following rules:
• If the hour value is less than 12, add the suffix a.m. ; otherwise, add the suffix p.m.
• If the hour value is greater than 12, subtract 12 from the value.
• If the hour value is equal to 0, change it to 12.