# 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

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