javascript

Chapter 3: Language Basics
Even though uninitialized variables are automatically assigned a value of
undefined, it is advisable to always initialize variables. That way, when typeof
returns “undefined”, you’ll know that it’s because a given variable hasn’t been
declared rather than simply not having been uninitialized.
The Null Type
The Null type is the second data type that has only one value: the special value null . Logically, a null
value is an empty object pointer, which is why typeof returns “object” when it ’ s passed a null value
in the following example:
var car = null;
alert(typeof car);
//”object”
When defining a variable that is meant to later hold an object, it is advisable to initialize the variable to
null as opposed to anything else. That way, you can explicitly check for the value null to determine if
the variable has been filled with an object reference at a later time, such as in this example:
if (car != null){
//do something with car
}
The value undefined is a derivative of null , so ECMA - 262 defines them to be superficially equal as follows:
alert(null == undefined);
//true
Using the equality operator ( == ) between null and undefined always returns true , though keep in
mind that this operator converts its operands for comparison purposes (covered in detail later in this
chapter).
Even though null and undefined are related, they have very different uses. As mentioned previously,
you should never explicitly set the value of a variable to undefined , but the same does not hold true for
null . Any time an object is expected but is not available, null should be used in its place. This helps to
keep the paradigm of null as an empty object pointer and further differentiates it from undefined .
The Boolean Type
The Boolean type is one of the most frequently used types in ECMAScript and has only two literal
values: true and false . These values are distinct from numeric values, so true is not necessarily equal
to 1, and false is not necessarily equal to 0. Assignment of Boolean values to variables is as follows:
var found = true;
var lost = false;
Note that the Boolean literals true and false are case-sensitive, so True and False (and other mixings
of uppercase and lowercase) are valid as identifiers but not as Boolean values.
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