javascript

Chapter 5: Reference Types
scope, whereas automatically created primitive wrapper objects exist for only one line of code before they
are destroyed. This means that properties and methods cannot be added at runtime. Take this for example:
var s1 = “some text”;
s1.color = “red”;
alert(sl.color); //undefined
Here, the second line attempts to add a color property to the string s1 . However, when s1 is accessed
on the third line, the color property is gone. This happens because the String object that was created
in the second line is destroyed by the time the third line is executed. The third line creates its own
String object, which doesn ’ t have the color property.
It is possible to create the primitive wrapper objects explicitly using the Boolean , Number , and String
constructors. This should be done only when absolutely necessary, because it is often confusing for
developers as to whether they are dealing with a primitive or reference value. Calling typeof on an
instance of a primitive wrapper type returns “ object ” , and all primitive wrapper objects convert to the
Boolean value true .
Even though it ’ s not recommended to create primitive wrapper objects explicitly, their functionality is
important in being able to manipulate primitive values. Each primitive wrapper type has methods that
make data manipulation easier.
The Boolean Type
The Boolean type is the reference type corresponding to the Boolean values. To create a Boolean object,
use the Boolean constructor and pass in either true or false as in the following example:
var booleanObject = new Boolean(true);
Instances of Boolean override the valueOf() method to return a primitive value of either true or
false . The toString() method is also overridden to return a string of “ true ” or “ false ” when
called. Unfortunately, not only are Boolean objects of little use in ECMAScript, they can actually be
rather confusing. The problem typically occurs when trying to use Boolean objects in Boolean
expressions, as in this example:
var falseObject = new Boolean(false);
var result = falseObject & & true;
alert(result); //true
var falseValue = false;
result = falseValue & & true;
alert(result); //false
In this code, a Boolean object is created with a value of false . That same object is then ANDed with the
primitive value true . In Boolean math, false AND true is equal to false . However, in this line of
code, it is the object named falseObject being evaluated, not its value ( false ). As discussed earlier, all
objects are automatically converted to true in Boolean expressions, so falseObject actually is given a
value of true in the expression. Then, true ANDed with true is equal to true .
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