Listing Factors of Numbers in JavaScript

Finding Prime Factors in JavaScript

Finding prime factors is all about selecting those factors of a number that are prime.

The prime factors of 36 as an example, are:

2 X 2 X 3 X 3.

Prime Factor code in JavaScript

We'll go about the code in a simple way:

Step 1:

Starting with 2, find the first factor of the number of interest - this factor will definitely be a prime.
Store this factor away.

Step 2:

Divide number in question by found factor.

Step 3:

Using result from Step 2 as the new number in question (number whose prime factors we are trying to find), repeat Step 1.

Step 4:

Continue recursively until we arrive at 1.

Step 5:

We'll use the square-root of number range.

Create a new file; On Notepad++: File, New.
Call it PrimeFactors.html.
Type out the adjoining JavaScript code for listing prime factors.

All those steps in a few lines of code;
Now functions come in handy, don't they?

JavaScript code for PrimeFactors.html

<!DOCTYPE html>

<html lang="en">
    <head>
        <meta charset="utf-8" />
        <title>Listing The Prime Factors Of A Number.</title>
    </head>
    <body>
        <h4>Prime factors constitutes those factors of a number that are prime.</h4>
        <div id="prime_factors"></div>

        <script>
            var find_my_factors = 48;
            var found_prime_factors = [];
            var count = 0; // index into array 'found_prime_factors'
            var i = 2; // variable for our function loop

            onlyPrimeFactors(find_my_factors); // Call our function

            /* If variable find_my_factors is a prime number(has no factors), we simply say so. */
            document.getElementById("prime_factors").innerHTML = found_prime_factors.length > 1 ?
            "The prime factors of " + find_my_factors + " are: <br />" + found_prime_factors.join(" X ") :
            find_my_factors + " has no other factors except 1 and itself. It is a prime number.";

            /*
            * Our function checks 'find_my_factors';
            * If it finds a factor, it records this factor,
            * then divides 'find_my_factors' by the found factor
            * and makes this the new 'find_my_factors'.
            * It continues recursively until all factors are found.
            */
            function onlyPrimeFactors(sub_level) {
                //STEP 5
                var temp_limit = Math.ceil(Math.sqrt(sub_level));

                //STEP 1
                while (i <= temp_limit) {
                    if (i != 1 && (sub_level % i) == 0) { // avoid an infinite loop with the i != 1 check.
                        found_prime_factors[count++] = i;
                        //STEPS 2, 3, 4
                        return onlyPrimeFactors(sub_level / i);
                    }
                    i++;
                }

                found_prime_factors[count] = sub_level;

                return;
            }
        </script>
    </body>
</html>

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