# Mathematics - Factorial

The factorial of a non-negative integer x is equal to the product of all integers less than or equal to x and greater than zero.

For example: factorial(4) would equal 4 * 3 * 2 * 1, which is 24.

In factorial, things are NOT repeated.

## Number of combination from a set

It's used to calculate the number of combination in a set that permits to get the chance or probability.

If you have 3 elements [ a b c ], you have 6 possibilities or factorial 3:

• a b c
• a c b
• b a c
• b c a
• c a b
• c b a

You have therefore 1 change over 6 to pick the good combination.

## Implementation

``````let factorial = function (n){
if(n === 0 || n === 1){
return 1;
} else {
return n * factorial(n-1);
}
}

console.log(factorial(3));```
```

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