Java Reference
In-Depth Information
Currently, its value is the
undefined
value because you've declared only its existence to the JavaScript
engine, not any actual data. It may sound odd, but
undefined
is an actual primitive value in JavaScript,
and it enables you to do comparisons. (For example, you can check to see whether a variable contains
an actual value or whether it has not yet been given a value, that is, whether it is undefined.) However,
in the next line you assign
myFirstVariable
a string value, namely the value
Hello
:
myFirstVariable
=
"Hello";
Here you have assigned the variable a
literal
value (that is, a piece of actual data rather than data
obtained by a calculation or from another variable). Almost anywhere that you can use a literal string
or number, you can replace it with a variable containing number or string data. You see an example
of this in the next line of code, where you use your variable
myFirstVariable
in the
alert()
function
that you saw in the previous chapter:
alert(myFirstVariable);
This causes the first
alert
box to appear. Next you store a new value in your variable, this time a number:
myFirstVariable
=
54321;
The previous value of
myFirstVariable
is lost forever. The memory space used to store the value is
freed up automatically by JavaScript in a process called
garbage collection
. Whenever JavaScript detects
that the contents of a variable are no longer usable, such as when you allocate a new value, it performs
the garbage collection process and makes the memory available. Without this automatic garbage
collection process, more and more of the computer's memory would be consumed, until eventually
the computer would run out and the system would grind to a halt. However, garbage collection is not
always as efficient as it should be and may not occur until another page is loaded.
Just to prove that the new value has been stored, use the
alert()
function again to display the
variable's new contents:
alert(myFirstVariable);
assigning variables with the value of other variables
You've seen that you can assign a variable with a number or string, but can you assign a variable with
the data stored inside another variable? The answer is yes, very easily, and in exactly the same way as
giving a variable a literal value. For example, if you have declared the two variables
myVariable
and
myOtherVariable
and have given the variable
myOtherVariable
the value
22
, like this:
var myVariable;
var myOtherVariable;
myOtherVariable
=
22;
you can use the following line to assign
myVariable
the same value as
myOtherVariable
(that is,
22
):
myVariable
=
myOtherVariable;
