JavaScript Core Objects - JavaScript by Example - page 264

EXAMPLE 9.33 ( CONTINUED )

document.write("The value of NaN is boolean " + bool5
+ "<br />");

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EXPLANATION
1 The argument passed to the Boolean object constructor is the initial value of the

object, either true or false . If the initial value is 0 , the empty string “” , NaN , or null ,
the result is false ; otherwise, the result is true .

2 The Boolean object's values are displayed as either true or false (see Figure 9.38).

Figure 9.38 True or False ? Output from Example 9.33.

9.5.7 The Function Object

The Function object lets you define a function as an object. It allows a string to be
defined at runtime and then compiled as a function. You can use the Function() con-
structor to create a variable that contains the function. Because the function has no
name, it is often called an anonymous function and its arguments are passed as comma-
separated strings. The last argument is the body of statements that will be executed
when the function is called. If the Function() constructor does not require arguments,
then the body of statements is treated as a string, and will be passed to the Function()
constructor to define what the function is to do. Because functions are objects, they also
have properties (see Table 9.13) and methods (see Table 9.14).
Function objects are evaluated each time they are used, causing them to be slower in
execution than normal JavaScript functions.

Table 9.13 Properties of the Function Object

Property What It Does

length Returns the number of arguments that are expected to be passed (read only).

prototype Allows the object to be customized by adding new properties and methods.