These are times when the same portion of code needs to
be executed many times with different values .
for
The for structure loops a preset number of times . The
for loop is made up of two parts the condition and statements
. The condition portion of the structure determines how
many times the loop repeats while the statement is what
is executed every time the loop occurs.
The conditional structure is contained within parentheses
and is made up of three parts , each separated by a semi
colon ( ; ) . The first part of the condition structure
initializes a variable to a starting value . In most cases
, the variable is declared here and initialized .
The second part is the actual conditional statement that
determines how many times the loop with be iterated . The
third and final part determines how the variable which was
initialized in the first part , should be changed each time
the loop is iterated . The third part causes the variable
to be incremented , decremented etc . Here is the syntax
of the for loop
for (initialize ; condition ; adjust)
{
statement ;
}
You have to beware of infinite loops . Lets look at a for
loop example .
<script language ="JavaScript">
<!--
for(var i = 0 ; i <= 10 ; i++)
{
document.write(" " , i );
}
//-->
</script>
which gives us the following result
while
The while loop is similar to the for loop . The statement
works like this while the expression in brackets is true
execute the statements in the loop . Once the last statement
is executed return to the top of the loop and evaluate the
expression again . When the expression evaluates to false
the next line of code after the while loop structure executes
. To prevent infinite loops a statement must be included
in the loop that modifies a variable in the expression .
Here is the syntax of the while loop .
while (expression)
{
statement;
}
Here is an example
<script language="JavaScript">
<!--
var i = 0;
while(i <= 10)
{
document.write(" " , i);
i++;
}
//-->
</script>
which gives us the following results
do while
This loop is simply a variant of the while loop . The important
difference is that the loop will always evaluate once before
evaluating the expression for the first time . Here is the
syntax.
do
{
statement ;
}
while (expression);
Once the loop has executed for the first time the expression
in brackets is evaluated . If this is true the loop is executed
once again . If this is false the next of line of code after
the while structure is executed . A statement must be included
within the loop that modifies a variable in the expression
to prevent infinite loops .
Here is the example above modified to use a do ... while
loop
<script language="JavaScript">
<!--
var i =0;
do
{
document.write(" " , i);
i++;
}
while(i <=10);
//-->
</script>
and here is the result again
for .. in
This is not the same as the for loop . The for...in loop
is used to provide access to the enumerated properties of
a JavaScript object . This loop is only found in JavaScript
. The statement in the loop are executed for each property
of an object until every property has been accessed. The
syntax looks like this
for ( variable in object)
{
statement ;
}
Here is an example that accesses all the properties of
the document object
<script language="JavaScript">
<!--
var i;
for(i in document)
{
document.write(" " ,i);
}
//-->
</script>
and here is the result
Break / Continue
Break is used to terminates execution of the innermost
enclosing loop and from JavaScript 1.2 onwards you can also
name the loop.
break ;
break label ;
Continue is restarts the innermost loop or from JavaScript
1.2 onwards can actually restart a named loop .
continue;
continue label;
