Finding the largest prime factor of a number in JavaScript

We are required to write a JavaScript function that takes in a number as the only argument.

The number provided as an argument is guaranteed to be a composite number (the number that has more than two factors). Our function should find the largest prime number that exactly divides the input number.

For example −

If the argument is 72, then the output should be 3.

Because 3 is the largest prime number that exactly divides 72

Example

Following is the code −

const num = 72;
const largestPrimeFactor = (num) => {
   let res = Math.ceil(Math.sqrt(num));
   const isPrime = (num) => {
      let i, limit = Math.ceil(Math.sqrt(num));
      for (i = 3; i <= limit; i += 2) {
         if (num % i === 0) {
            return false;
         };
      };
      return true;
   };
   res = (res & 1) === 0 ? res - 1 : res;
   while (!(num % res === 0 && isPrime(res))) {
      res -= 2;
   };
   return res;
}
console.log(largestPrimeFactor(num));

Output

Following is the output on console −

3