# Function to compute factorial of a number in JavaScript

In this article, the given task is to get the factorial of a number in JavaScript.

### What is the factorial of a number?

The multiplication of all positive integers smaller than or equal to n gives the factorial of a non-negative integer.

For example, assume the non-negative integer is 4 and the factorial of 4 will be 4*3*2*1 which is equal to 24. The symbol of factorial is represented by "!". so it will be (4!).

The value of 0! Will be 1 as 0 is not a positive integer.

### The mathematical formula of factorial

("n!") This is how the factorial of a number is represented. it is the product of all positive integers which are less than or equal to n.

Below is the formula of factorial.

n! = n*(n-1)*(n-2)*……3*2*1


## Using for loop

We can calculate the factorial of a number by using for loop.

### Algorithm

These are the steps to get the factorial of a given number by using for loop.

• create a variable (result) with value 1.

• Call the function fact(num).

• If the num is less than 0, will return -1.

• If the num = 0, will return 1.

• If the num > 1, with each iteration it will decrease the value by 1 and multiply by the result.

• The factorial result will be stored in the result variable.

### Example

Following is the program to get the factorial of a number by using the above for loop algorithm.

<!DOCTYPE html>
<html>
<head>
<title>Factorial of a number</title>
<button onClick = "func()"> Click to get factorial </button>
<p id="para"> </para>
<script>
function func() {
function fact(num) {
if (num < 0){
return -1;
}
else if(num == 0){
return 1;
}
else {
let result = 1;
for(var i = num; i > 1; i--){
result *= i;
};
return result;
}
};
const num = 4;
document.getElementById("para").innerHTML = fact(num);
};
</script>
</head>
</html>


As we can see in the output, it calculated the factorial of a number with the help of for loop by iterating the input number and decreasing it for every iteration.

## Using recursion

We can get the factorial of a number by recursion.

### Algorithm

Below are the following steps to achieve factorial of number by recursion.

• Get the number from num.

• Call the function factorial(num).

• If num is less than 0, then return -1.

• Else if num = 0 return 1.

• Else, return (num * factorial(num - 1))

### Example

Following is the example of getting the factorial of a number by using the above recursive algorithm −

<!DOCTYPE html>
<html>
<title>Factorial of a number</title>
<head>
<button onClick = "func()">Factorial</button>
<p id="para"> </para>
<script>
function func(){
function factorial(num) {
if (num < 0) {
return -1;
}
else if (num == 0){
return 1;
}
else {
return num * factorial(num - 1);
}
}
let num = 4;
document.getElementById("para").innerHTML = factorial(num);
};
</script>
</head>
</html>


On executing the above program a button named Factorial is displayed. If you click on this button The factorial of the input number (4) is calculated and the result is printed.

## Using While loop

We can achieve the factorial of a number by using while loop.

### Algorithm

These are steps to calculate the factorial of a number by using while loop.

• create variable res.

• Call the function fact(num).

• If the num is less than 0, will return -1.

• If the num = 0, will return 1.

• If the num > 1, with each iteration it will decrease the value by 1 and multiply by the result.

• The factorial result will be stored in the result variable.

### Example

Following is the example of getting the factorial of a number by using a while loop −

<!DOCTYPE html>
<html>
<head>
<title>Factorial of a number</title>
<button onClick = "func()">Click me! </button>
<p id = "para"></para>
<script>
function func(){
function fact(num) {
var res = num;
if (num === 0){
return 1;
}
else if (num === 1){
return 1;
}
else{
while (num > 1) {
num--;
res *= num;
}
}
return res;
};
const num = 4;
document.getElementById("para").innerHTML = fact(num);
};
</script>
</head>
</html>


As we can see from the output, the while loop was used to calculate the factorial of a given number by iterating the input number and decreasing it each time.

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