Find all palindromes made from the product of two 4-digit numbers

index.js

const fs = require('fs');
const reverseNum = s => String(s).split('').reverse().join('');
const genPalindrome = (len) => {
    const palindromes = new Set();
    const mod = len%2 !== 0;
    const iLen = ~~(len/2);
    const start = Math.pow(10, iLen - 1);
    const end = Math.pow(10, iLen) - 1;
    
    for (let i = end; i >= start; i--)
    {
        if (mod) {
            for (let j = 9; j>=0; j--) {
                palindromes.add( Number(i + String(j) + reverseNum(i)) );
            }
        } else {
            palindromes.add( Number(i + reverseNum(i)) );
        }
    }
    return palindromes;
};

const palindromes7 = genPalindrome(7);
const palindromes8 = genPalindrome(8);
const palindromes = new Set([...palindromes7, ...palindromes8]);

let res = '';
for (let i = 9999; i > 1000; --i) {
    if (i%10 === 0) continue;
    for (let j = i, mult = i*j; j > 1000; --j, mult -= i) {
        if (i%10 === j) continue;
        if (palindromes.has(mult)) {
            res += `${i} ${j} ${mult}\n`;
        }
    }
}

fs.writeFile('output.txt', res, () => process.exit()); 

Intel Core I5-4300M CPU, 16 GB RAM

real    0m1.657s
user    0m0.000s
sys     0m0.078s

https://gist.github.com/realsba/e89641d762db3b5cc0178afc3daa3d97

https://gist.github.com/realsba/eee0cefc2ffd7f2a078bb8293e5b8a8a

https://gist.github.com/realsba/62aa1192efb47c6f9a1bb6dec31ea7db

Java implementation https://gist.github.com/belichuk/672683085f510f7c772cf5ee75160b36