https://leetcode.com/explore/challenge/card/30-day-leetcoding-challenge/528/week-1/3284/

happyNumber.js

/**
 * @param {number} n
 * @return {boolean}
 */
var isHappy = function(n) {
    
    const seen = new Set();
    
    const next = (n) => {
        let sum = 0;
        while(n){ // keep popping off the last digit until we get to zero
            const r = n%10;
            n = Math.floor(n/10);
            sum+=Math.pow(n, 2);
        }
        return sum;
    };
    
    while(n !== 1){ // search for cycle
        if(seen.has(n)) return false;
        seen.add(n);
        n = next(n);
    }
    return true;
};