Math Buddies

collatz.js

function collatz(n, steps = []) {
    if (isNaN(n)) return NaN;
    if (steps.length === 0) steps.push(n);

    const isEven = n%2 === 0;
    const newInt = isEven ? n / 2 : (3 * n) + 1;
        
    steps.push(newInt);
    
    return newInt !== 1 ? collatz(newInt, steps) : steps;
    
}

Math.collatz = collatz;

factorial.js

function factorial(n) {
  if (isNaN(n)) return NaN;
  let result = n;
  
  while (n > 1) result = result * --n;
  
  return result;
}

Math.factorial = factorial;

format.js

function format (n, separator = ',') {
    if (isNaN(n) ) return NaN;
    const nString = n.toString();
    const nCommas = [...nString].reverse().reduceRight((acc, cur, idx) => {
        const needsSeparator = (idx + 1)%3 === 0 && (idx + 1 < nString.length);
        return `${acc}${needsSeparator ? separator : '' }${cur}`;
    }, '');

    return nCommas;
}

Math.format = format;

isPrime.js

function isEven(n) {
    return n % 2 === 0
}

function isPerfectSquare(n) {
    return Math.ceil(Math.sqrt(n)) === Math.sqrt(n)
}

function getRangeOfTestables(n) {
    const range = [];
    let rangeCounter = 0;

    /* don't need get  range from 0 to [n]: 
        once you get to (n / 3) + 1, it's not possible for [n] to be a multiple of that number:
        Take range of integers from 0-53: [3,5,7,11,13,15,17,19,21,23,27,29,31,33,35,37,39,41,43,45,47,49,51]
        3 * 17 == 51
        3* 19 == 57
        BTW: could filter just the first _third_ of prime numbers, instead. But Recursion slows this down
    */ 
    while (rangeCounter++ < Math.ceil(n / 3)) {
        if (!isEven(rangeCounter) && !isPerfectSquare(rangeCounter)) range.push(rangeCounter);
    }

    /*
        Don't need last el of the range because lastEl * firstEl == n
    */
    return range.slice(0, range.length - 1);
}

function isPrime(n) {
    // return early if it doesn't meet easy-to-recognize criteria for being prime
    if (isNaN(n) || isEven(n) || isPerfectSquare(n)) return false;

    let isPrime = true;
    const rangeOfTestables = getRangeOfTestables(n);

    for (let int of rangeOfTestables) {
        if (n % int === 0) {
            isPrime = false;
            break;
        }
    }

    return isPrime;
}

Math.isPrime = isPrime;