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In any programming language there are a lot of available looping statements, where
we can execute our statements for one or more times. We can do this though the use of loops
here in JavaScript.
Now in JavaScript will guide us that the concept of looping is based on the
iterations, and on each iteration some values are changed till the
loop executes. JavaScript loop is a circle, and this circle repeats itself
till we want, and this activity when call or recall one time then its called one
iteration. The JavaScript loops are
typically controlled by a test of a
JavaScript variable, and it is that major variable which control the iterations
of loop, because if we don't do nothing with the control of JavaScript Loop, it
will be an infinite
loop which does the same thing again and again, and an infinite loop never stops. That's not good
in programming approach, and it may be the answer why our JavaScript hangs for a long time
and it produces no result, the reason would be the infinity of JavaScript loop only...
In JavaScript there are four types of loops as below.
1 >> JavaScript for loop
2 >> JavaScript for...in loop
3 >> JavaScript while loop
4 >> JavaScript do...while loop
The easiest one is the JavaScript for statement. It specifies a counter
JavaScript
variable which will use for a test condition, and an action that updates the
counter before each iteration.
Syntax of for loop in JavaScript
for (variable_initialization; expression; increment){
JavaScript statements
}
On above
"for" executes the JavaScript statements as long as the specified and expression is evaluated to "True"
till the life of JavaScript loop. The following example fills an array with
numbers from 0 to 6.
Example of JavaScript for loop
<script language="javascript">
var day = new Array();
for (i = 0; i < 7; i++) //as long as the variable "i"
is smaller than 7
{
day[i] = i;
}
</script>
Syntax of for in loop in JavaScript
JavaScript For---In loop is a special kind of loop structure in
JavaScript. JavaScript uses it for stepping through all the elements of an
array, or all the user-defined properties of an object. The syntax is a little
different from the previous one
<script language="javascript">
for (variable in [object | array]){
JavaScript statements
}
</script>
Now if we want a more open advance to JavaScript loops, we can always use the while
structure. There is a slight difference between JavaScript for loop and
JavaScript while loop, when
we use JavaScript while, we must do all the counter
JavaScript variable initialization and variation
ourself. "while" will only accept a expression, and if the expression is evaluated
to "True", then it will execute the JavaScript statements we provide. Because
JavaScript while loops don't
have an explicit built-in counter JavaScript variable, they are more exposed to
JavaScript infinite loops,
so always remember to change the counter-variable's value inside the JavaScript loop! Here is the
same JavaScript loop example as before, but we use JavaScript "while" instead of
JavaScript "for" here in JavaScript Tutorial.
Syntax of while loop in JavaScript
<script language="javascript">
var day = new Array();
i = 0; //we have to do the initialization ourselves here
while(i < 7)
{
day[i] = i;
i++;
//don't forget to alter the counter-variable's value,
//or else we'll get an infinite loop here
}
</script>
Similar to the JavaScript while loop is the JavaScript do...while loop, which actually performs
the test after it executes the loop. This guarantees that the JavaScript loop is executed
at least once, after which it may or may not continue, based on the evaluation
of the expression provided with while loop.
Syntax of do while loop in JavaScript
<script language="javascript">
var numbers = new Array();
i = 0;
do
{
numbers[i] = i; //this will parse at least once...
i++; //this will also parse at least once
}
while (i < 10);
</script>
There are times when we encounter special values within a JavaScript loop, and
we may want
to simple exit the loop, or continue to the next iteration of our loop. JavaScript
provides two such statements one is "break" and second is "continue". The
JavaScript break statement immediately stops the execution of any loop, while the
JavaScript continue statement stop the current iteration only, and starts the
next one. No matter which statement we choose to use, the previous statements before
JavaScript
break statement or JavaScript continue statement are always executed, but the
below ones will not be parsed.
We can use JavaScript break statement and JavaScript continue statement with any of the
JavaScript "for", "while" or "do...while" loops.
Syntax of breaking any loop in JavaScript
<script language="javascript">
var day = new Array();
for (i = 0; i < 7; i++) //as long as the variable "i" is smaller than 7...
{
if(i == 4)
continue; //let's skip the number 5
numbers[i] = i;
}
</script>
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