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JavaScript Loop Control


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JavaScript provides you full control to handle your loops and switch statement. There may be a situation when you need to come out of a loop without reaching at its bottom. There may also be a situation when you want to skip a part of your code block and want to start next iteration of the look.

To handle all such situations, JavaScript provides break and continue statements. These statements are used to immediately come out of any loop or to start the next iteration of any loop respectively.

The break Statement:

The break statement, which was briefly introduced with the switch statement, is used to exit a loop early, breaking out of the enclosing curly braces.

Example:

This example illustrates the use of a break statement with a while loop. Notice how the loop breaks out early once x reaches 5 and reaches to document.write(..) statement just below to closing curly brace:

<script type="text/javascript">
<!--
var x = 1;
document.write("Entering the loop<br /> ");
while (x < 20)
{
  if (x == 5){ 
     break;  // breaks out of loop completely
  }
  x = x + 1;
  document.write( x + "<br />");
}
document.write("Exiting the loop!<br /> ");
//-->
</script>

This will produce following result:

Entering the loop
2
3
4
5
Exiting the loop!

To understand it in better way you can Try it yourself.

We already have seen the usage of break statement inside a switch statement.

The continue Statement:

The continue statement tells the interpreter to immediately start the next iteration of the loop and skip remaining code block.

When a continue statement is encountered, program flow will move to the loop check expression immediately and if condition remain true then it start next iteration otherwise control comes out of the loop.

Example:

This example illustrates the use of a continue statement with a while loop. Notice how the continue statement is used to skip printing when the index held in variable x reaches 5:

<script type="text/javascript">
<!--
var x = 1;
document.write("Entering the loop<br /> ");
while (x < 10)
{
  x = x + 1;
  if (x == 5){ 
     continue;  // skill rest of the loop body
  }
  document.write( x + "<br />");
}
document.write("Exiting the loop!<br /> ");
//-->
</script>

This will produce following result:

Entering the loop
2
3
4
6
7
8
9
10
Exiting the loop!

To understand it in better way you can Try it yourself.

Using Labels to Control the Flow:

Starting from JavaScript 1.2, a label can be used with break and continue to control the flow more precisely.

A label is simply an identifier followed by a colon that is applied to a statement or block of code. We will see two different examples to understand label with break and continue.

Note: Line breaks are not allowed between the continue or break statement and its label name. Also, there should not be any other statement in between a label name and associated loop.

Example 1:

<script type="text/javascript">
<!--
document.write("Entering the loop!<br /> ");
outerloop:   // This is the label name
for (var i = 0; i < 5; i++)
{
  document.write("Outerloop: " + i + "<br />");
  innerloop:
  for (var j = 0; j < 5; j++)
  {
     if (j >  3 ) break ;         // Quit the innermost loop
     if (i == 2) break innerloop; // Do the same thing
     if (i == 4) break outerloop; // Quit the outer loop
     document.write("Innerloop: " + j + "  <br />");
   }
}
document.write("Exiting the loop!<br /> ");
//-->
</script>

This will produce following result:

Entering the loop!
Outerloop: 0
Innerloop: 0 
Innerloop: 1 
Innerloop: 2 
Innerloop: 3 
Outerloop: 1
Innerloop: 0 
Innerloop: 1 
Innerloop: 2 
Innerloop: 3 
Outerloop: 2
Outerloop: 3
Innerloop: 0 
Innerloop: 1 
Innerloop: 2 
Innerloop: 3 
Outerloop: 4
Exiting the loop!

To understand it in better way you can Try it yourself.

Example 2:

<script type="text/javascript">
<!--
document.write("Entering the loop!<br /> ");
outerloop:   // This is the label name
for (var i = 0; i < 3; i++)
{
   document.write("Outerloop: " + i + "<br />");
   for (var j = 0; j < 5; j++)
   {
      if (j == 3){
         continue outerloop;
      }
      document.write("Innerloop: " + j + "<br />");
   } 
}
document.write("Exiting the loop!<br /> ");
//-->
</script>

This will produce following result:

Entering the loop!
Outerloop: 0
Innerloop: 0
Innerloop: 1
Innerloop: 2
Outerloop: 1
Innerloop: 0
Innerloop: 1
Innerloop: 2
Outerloop: 2
Innerloop: 0
Innerloop: 1
Innerloop: 2
Exiting the loop!

To understand it in better way you can Try it yourself.



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