The do .... while loop is like a while loop, except that the loop expression is tested at the end of the loop rather than at the beginning. This means that the body of the loop is always executed at least once.
The syntax is:
do
{
body of the loop
} while (expression);
An important difference between while and do..while statement is in the do..while loop body of the loop always executed at least once before the condition can be executed. In a while loop, first condition will be evaluated and then only based on the result of the condition the body of the loop will be executed or not.
Example:
< Htrnl >
< Head >
< Title > Program - To test do..while
statement in JavaScript  < /Title >
< /Head >
< Body >
< script language="javascript"
type= "text/javascri pt" >
document.write("< h2 > Using do..while
Statement < /h2 >'');
var no2=0;
do
{
document. write( no2+" ");
no2=no2+2;
}while(no2<=10);
< /script >
< /Body >
< /Html >

< Html >
< Body >
< script language="javascript" type="text/ javascript" >
var N=0;
var N = promt ("Enter the number","0");
if(N%2==0)
{
alert ("The Number is Even").
}
else {
alert ("The Number is Odd")
}
< /script >
< /body >
< /Html >

< Html  >
< Body >
< script . 'language="javascript" type="text/ javascript" >
var N=0;
var N = promt ("Enter the number","0");
if(N > 0)
{
alert ("The Number is Positive ")
}
else {
alert ("The Number is Negative")
}
< /script >
< /body >
< /Html >

The do ..while loop is like a while loop, except that the loop expression is tested at the end of the loop rather than at the beginning. This means that the body of the loop is always executed a,t least once. The syntax is:
do
{
body of the loop
} while (expression);
An important difference between while and do ..
while statement is in the do ..while loop
body of the loop always executed at least once before the condition can be executed. In a while loop, first condition will be evaluated and then only based on the result of the condition the body of the loop will be executed or not.
Using do ..while loop
< Html >
< Head >
< Title >Program - To test do ..while statement in
JavaScript < /Title >
< /Head >
< Body >
< script language="javascript" type="text/ javascript" >
document.write("< h2 > Using do ..while
Statement < /h2 >");
var no2=0; do
{
document. write(no2+"");
no2=no2+2;
}while(no2< = 10);
< /script >
< /Body >
< /Html >

In JavaScript while loop is another most basic loop. The purpose of a while loop is to execute a  statement /block of statement repeatedly as long as an expression is true.
The while statement creates a loop that executes a specified statement as long as the test condition evaluates to true. The condition is evaluated before executing the statement.
The syntax is:
while (condition)
{
body of the loop
}
Using while loop:
< Html >
< Head >
< Title >Program - To test while statement in
JavaScript < /Title >
< /Head >
< Body >
< script language=;"javascript" type="text/ javascript" >
document.write("< h2 > Using while Statement < /h2 >");
var no2=0; while(no2< =5)
{
document. write(no2+"");
no2=no2+1;
}
< /script >
< /Body >
< /Html > 

To execute a" while statement, the interpreter first evaluates expression. If the value of the expression is true the interpreter executes the statement and repeats, jumping back to the top of the loop and evaluating expression again. In the above example let us see how the while command is executed,
1. Initial value of the variable no2 is set to 0
2. The expression in the while statement IS executed.
3. If the condition is true then body of the loop will be executed once otherwise body of the loop will be skipped and control will jump to next statement to end of the loop.
4. Then the value of the control variable is executed and control jumps to condition again go to step 3.