#### Permutation And Combination

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E
```/**
* @details Calculates the number of permutations and combinations.
*/

/**
* @brief Calculates the factorial of the given number.
* @param num: integer
* @details Factorial of n = n * (n - 1) * (n - 2) * ... * 1
* @returns integer: Factorial of the number.
NaN: if negative number is provided.
*/
const factorial = (n) => {
if (n >= 0) {
if (n === 0) {
return 1
} else {
return n * factorial(n - 1)
}
} else {
return NaN
}
}

/**
* @brief Calculates the number of Permutations from the given data.
* @param
* n: integer -> number of items.
* r: integer -> number of times n is taken.
* @returns integer: The number of permutations.
NaN: if negative number is provided.
*/
const permutation = (n, r) => {
return factorial(n) / factorial(n - r)
}

/**
* @brief Calculates the number of Combinations from the given data.
* @param
* n -> number of items.
* r -> number of times n is taken.
* @returns integer: The number of combinations.
NaN: if negative number is provided.
*/
const combination = (n, r) => {
return factorial(n) / (factorial(r) * factorial(n - r))
}

// Exports the functions to be used in other files.
export { factorial, permutation, combination }
```  