javascript

Chapter 3: Language Basics
If you ’ re not sure that a value isn ’ t null or undefined , you can use the String() casting function,
which always returns a string regardless of the value type. The String() function follows these rules:
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If the value has a toString() method, it is called (with no arguments) and the result is
returned.
If the value is null , “null” is returned.
If the value is undefined , “undefined” is returned.
Consider the following:
var value1 = 10;
var value2 = true;
var value3 = null;
var value4;
alert(String(value1)); //”10”
alert(String(value2)); //”true”
alert(String(value3)); //”null”
alert(String(value4)); //”undefined”
Here, four values are converted into strings: a number, a Boolean, “null” , and “undefined” . The result
for the number and the Boolean are the same as if toString() were called. Because toString() isn ’ t
available on “null” and “undefined” , the String() method simply returns literal text for those
values.
The Object Type
Objects in ECMAScript start out as nonspecific groups of data and functionality. Objects are created by
using the new operator followed by the name of the object type to create. Developers create their own objects
by creating instances of the Object type and adding properties and/or methods to it, as shown here:
var o = new Object();
This syntax is similar to Java, although ECMAScript requires parentheses to be used only when
providing arguments to the constructor. If there are no arguments, as in the following example, then the
parentheses can be omitted safely (though that ’ s not recommended):
var o = new Object; //legal, but not recommended
Instances of Object aren ’ t very useful on their own, but the concepts are important to understand
because, similar to java.lang.Object in Java, the Object type in ECMAScript is the base from which
all other objects are derived. All of the properties and methods of the Object type are also present on
other, more specific objects.
Each Object instance has the following properties and methods:
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constructor — The function that was used to create the object. In the previous example, the
constructor is the Object() function.
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