# Prime number program in JavaScript

Write the javascript program display whether a number is prime or not.

The Prime number is a whole number that will divide by 1 or itself is called a prime number.

## Prime number program in JavaScript using for loop

In this program, you will check whether a given number is prime or not in the JavaScript program using for loop

``````<!DOCTYPE html>
<html>
<body>
<script>
function PrimeNo()
{
var i,flag=0;
var n=prompt("enter a number");
for(i=1;i<=n;i++)
{
if(n%i==0)
{
flag+=1;
}
}
if(flag==2)
{ document.write(n+" is prime number"); }

else{ document.write(n+" is not a prime number"); }
}
</script>
<form>
<input type="button" value="prime number" onclick="PrimeNo();">
</form>
</body>
</html>
``````

### Explanation of checking whether a number is prime or not:

• When you click on the prime number button, `PrimeNo()` function will get called.
• In the `PrimeNo()` function, you will take input from the user using the prompt method, and you will store the input value in variable `n`.
• Now, iterate the for loop from 1 to n, i.e ` for(i=1; i<=n;i++) `, and when input value n is divisible by exactly two times during an iteration of the for-loop that means n is a prime number.
• `Flag` variable is used for counting how many times an input value `n` is divisible.
• If the `flag` value is 2 then the number is prime otherwise the number is not prime.

## JavaScript program to check prime number using form input

This program is the same as the above program only difference is here you have taken input through the HTML input box.

``````<html>
<body>
<form name="f1">
enter the number <input type="number" name="no">
<input type="button" value="output" onclick="PrimeNo()" value="output">
</form>
<p id="prime">output here</p>
<script>
function PrimeNo()
{
var i,flag=0;
var p=" ";
var a=parseInt(document.f1.no.value);
for(i=1;i<=a;i++)
{
if(a%i==0)
{
flag+=1;
}
if(flag==2)
{p=a+" is prime number"; }

else{  p=a+" is not a prime number";}
}
var d=document.getElementById("prime").innerHTML=p;

}
</script>
</body>
</html>``````

Click on the 'run now' button to view the output

• In this program, will take input from the HTML input box, and when you click on output button, PrimeNo() function will get called.
• PrimeNO() function will detect whether the entered number is prime or not
• if the number is prime then the output will display, the number is prime with the help of innerHTML. i.e `var d=document.getElementById("prime").innerHTML=p; `.
• InnerHTML will replace the content of paragraph which contains id prime,
``<p id="prime">output here</p>``
when you run the program, in place of 'output here ' variable p value will display.