JAVASCRIPT – FOR LOOP

Introduction

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 executed 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...

JavaScript for…in loop

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 (variable_name 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...