frente.js

function decode(arr) {
    const singleStrArr = arr.join('').split('')
  return singleStrArr
      .filter((i, j) => 
        !Number(i) 
        && Number(singleStrArr[j + 1]))
      .join('')
}

verso.js

const code = [
   'O3b4udfr9mklmi2g1a8ddd',
   '912io2eedp88o8r',
   '14f7a2z8e4rrr133p7a8j',
   'jtr5wnt500de2d13a52m3i',
   '1n3h1aaaaaa8v9i11ddd9kl',
   'jna1,1t4e2aaaa9mdsadasm5o1.1'
]

console.log(decode(code));