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Unit 5: Scripting Language
5.6 JavaScript Loops Notes
Often when you write code, you want the same block of code to run over and over again in a
row. Instead of adding several almost equal lines in a script we can use loops to perform a task
like this.
In JavaScript, there are two different kinds of loops:
for - loops through a block of code a specified number of times
while - loops through a block of code while a specified condition is true
5.6.1 The for Loop
The for loop is used when you know in advance how many times the script should run.
Some important points to note are:
The initialization statements are executed once; only when the for loop is encountered.
After execution of initialization statements, the condition is evaluated.
After every iteration, the updation statements are executed and then the condition is
checked.
Syntax:
for (var=startvalue;var<=endvalue;var=var+increment)
{
code to be executed
}
Example: Loop example.
The example below defines a loop that starts with i=0. The loop will continue to run as long as
i is less than, or equal to 5. i will increase by 1 each time the loop runs.
Notes The increment parameter could also be negative, and the <= could be any comparing
statement.
<html>
<body>
<script type=”text/javascript”>
var i=0;
for (i=0;i<=5;i++)
{
document.write(“The number is” + i);
document.write(“<br />”);
}
</script>
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