Valid Palindrome

The problem

A phrase is a palindrome if, after converting every uppercase letter to lowercase and removing all non-alphanumeric characters, it reads the same forwards and backwards.

Given a string `s`, return whether it's a palindrome. An empty string after cleaning counts as one, since it reads the same either way.

The approach

My idea is to first form a string with nothing but the alphanumeric characters, lowercasing as I go. A small `isAlphanumeric` helper tests each character with a regex, and I append the ones that pass to `final_string`.

Then it's a plain palindrome check: a `reverseString` helper splits, reverses, and re-joins the cleaned string, and I return whether `final_string` equals its reverse.

The solution

/**
 * @param {string} s
 * @return {boolean}
 */

const isAlphanumeric = (input) => /^[a-z0-9]+$/gi.test(input)
const reverseString = (input) => input.split("").reverse().join("")

var isPalindrome = function(s) {
    let final_string = ""
    for (el of s) {
        if (isAlphanumeric(el)) {
            final_string += String(el).toLowerCase()
        }
    }
    return final_string == reverseString(final_string)
};

Time O(n)Space O(n)