# Find factorial number program in JavaScript

In this program, the user will take input through the window prompt and find the factorial numbers in JavaScript.

factorial of a number is the multiplication of all its below number. for example: 5! = 5*4*3*2*1 = 120

## Factorial program in JavaScript using for loop

this program will find factorial of numbers by using for loop

``````<!DOCTYPE html>
<html>
<body>
<script>
function factorial()
{
var fact=1,i;
var a=prompt("enter a number:");
for(i=1;i<=a;i++)
{
fact=fact*i;
}
document.write("factorial of number "+a+" is:",fact);
}
</script>
<form>
<input type="button" value="factorial" onclick="factorial();">
</form>
</body>
</html>
``````

Input has taken through javascript prompt method

## output

```enter a number: 4
factorial of number 4 is:24
```

### Logic to find factorial of the number

• Declared` fact` variable with initialization value 1.
• Take a number from the user and store it in a variable `a`.
• Iterate the loop from 1 to user value i.e `a`,
• inside the loop calculate the factorial of a given number by using code   `fact=fact*i;`
• when the loop breaks factorial value will get stored in a `fact` variable, after the loop end print the factorial value.

## Find Factorial program in JavaScript using while loop

In this example, you will take input from the user by using window prompt method, and find the factorial of number using the while loop.

``````<!DOCTYPE html>
<html>
<body>
<script>
function factorial()
{
var fact=1,i=1;
// take input from user using prompt
var a=prompt("enter a number:");
while(i<=a)
{
// calculate factorial of number
fact=fact*i;
i++;
}
// print the factorial value
document.write("factorial of number "+a+" is:",fact);
}
</script>
<form>
<input type="button" value="factorial" title="calculate factorial" onclick="factorial();">
</form>
</body>
</html>
``````

Input has been taken through the javascript prompt method

## output

```enter a number: 5
factorial of number 5 is:120
```

## Find Factorial of number in JavaScript by using do while loop

In this example, you will take number from the user by using window prompt method, and find the factorial of number by using the do while loop.

``````<!DOCTYPE html>
<html>
<body>
<script>
function factorial()
{
var fact=1,i=1;
// take input from user
var a=prompt("enter a number:");
do
{
// calculate factorial
fact=fact*i;
i++;
}while(i<=a);
// print factorial value
document.write("factorial of number "+a+" is:",fact);
}
</script>
<form>
<input type="button" value="factorial" onclick="factorial();">
</form>
</body>
</html>

``````

Input has been taken through the javascript prompt method

## output

```enter a number: 3
factorial of number 3 is:6
```