JavaScript Loops | Types of Loops

A loops causes a specific block of statements to be repeated a specific number of time as long as some condition remains true.

  • While loop
  • Do-while loop
  • For loop
  • For IN

JavaScript supports all the necessary loops to ease down the pressure of programming.

The while Loop

The most basic loop in JavaScript is the while loop which would be discussed in this chapter. The purpose of a while loop is to execute a statement or code block repeatedly as long as an expression is true. Once the expression becomes false, the loop terminates.

Flow Chart

The flow chart of while loop looks as follows:

Syntax
The syntax of while loop in JavaScript is as follows-

while (expression) {
Statement(s) to be executed
if expression is true
}

Example
Try the following example to implement while loop.

<html>
 <body>
 
 <script type =
"text/javascript">
 <!--
 var count = 0;
 
document.write("Starting Loop 
");
 
 while (count < 10) {
 
document.write("Current Count : 
" + count + "<br />");
 count++;
 }
 
 document.write("Loop 
stopped!");
 //-->
 </script>
 
 <p>Set the variable to 
different value and then 
try...</p>
 </body>
</html>

Output

Starting Loop
Current Count : 0
Current Count : 1
Current Count : 2
Current Count : 3
Current Count : 4
Current Count : 5
Current Count : 6
Current Count : 7
Current Count : 8
Current Count : 9
Loop stopped!
Set the variable to different 
value and then try...

The do…while Loop

The do…while loop is similar to the while loop except that the condition check happens at the end of the loop. This
means that the loop will always be executed at least once, even if the condition is false.

Flow Chart

The flow chart of a do-while loop would be as follows

Syntax
The syntax for do-while loop in JavaScript is as follows −

do {
 Statement(s) to be executed;
} while (expression);

Note − Don’t miss the semicolon used at the end of the do…while loop.

Example
Try the following example to learn how to implement a do-while loop in JavaScript

<html>
 <body> 
 <script type =
"text/javascript">
 <!--
 var count = 0;
 
 
document.write("Starting Loop" +
"<br />");
 do {
 
document.write("Current Count : 
" + count + "<br />");
 count++;
 }
 
 while (count < 5);
 document.write 
("Loop stopped!");
 //-->
 </script> 
 <p>Set the variable to 
different value and then 
try...</p>
 </body>
</html>

Output

Starting Loop
Current Count : 0
Current Count : 1
Current Count : 2
Current Count : 3
Current Count : 4
Loop Stopped!
Set the variable to different value and then try…

For loop

The ‘for’ loop is the most compact form of looping. It includes the following three important parts

  • The loop initialization where we initialize our counter to a starting value. The initialization statement is execute before the loop begins
  • The test statement which will test if a given condition is true or not. If the condition is true, then the code given inside the loop will be executed, otherwise the control will come out of the loop.
  • The iteration statement where you can increase or decrease your counter.

You can put all the three parts in a single line separated by semicolons

Flow Chart

The flow chart of a for loop in JavaScript would be as follows

Syntax
The syntax of for loop is JavaScript is as follows −

for (initialization; test 
condition; iteration statement) 
{
 Statement(s) to be executed 
if test condition is true
}

Example
Try the following example to learn how a for loop works in JavaScript.

<html>
 <body> 
 <script type =
"text/javascript">
 <!--
 var count;
 
document.write("Starting Loop" +
"<br />");
 
 for(count = 0; count 
< 10; count++) {
 
document.write("Current Count : 
" + count );
 
document.write("<br />");
 } 
 document.write("Loop 
stopped!");
 //-->
 </script> 
 <p>Set the variable to 
different value and then 
try...</p>
 </body>
</html>

Output

Starting Loop
Current Count : 0
Current Count : 1
Current Count : 2
Current Count : 3
Current Count : 4
Current Count : 5
Current Count : 6
Current Count : 7
Current Count : 8
Current Count : 9
Loop stopped!
Set the variable to different
value and then try…

For in

The for…in loop is used to loop through an object’s properties. As we have not discussed Objects yet, you may not feel
comfortable with this loop. But once you understand how objects behave in JavaScript, you will find this loop very useful.

Syntax
The syntax of ‘for. In’ loop is −

for (variablename in object) {
 statement or block to execute
}

In each iteration, one property from object is assigned to variable name and this loop continues till all the properties of the object are exhausted.
Example
Try the following example to implement ‘for-in’ loop. It prints the web browser’s Navigator object.

<html>
 <body> 
 <script type =
"text/javascript">
 <!--
 var aProperty;
 
document.write("Navigator Object 
Properties<br /> "); 
 for (aProperty in 
navigator) {
 
document.write(aProperty);
 
document.write("<br />");
 }
 document.write 
("Exiting from the loop!");
 //-->
 </script> 
 <p>Set the variable to 
different object and then 
try...</p>
 </body>
</html>

Output

Navigator Object Properties 
serviceWorker 
webkitPersistentStorage 
webkitTemporaryStorage 
geolocation 
doNotTrack 
onLine 
languages 
language 
userAgent 
product 
platform 
appVersion 
appName 
appCodeName 
hardwareConcurrency 
maxTouchPoints 
vendorSub 
vendor 
productSub 
cookieEnabled 
mimeTypes 
plugins 
javaEnabled 
getStorageUpdates 
getGamepads 
webkitGetUserMedia 
vibrate 
getBattery 
sendBeacon 
registerProtocolHandler 
unregisterProtocolHandler 
Exiting from the loop!
Set the variable to different 
object and then try...
JavaScript Loops | Types of Loops
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