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function dec(strIn, token) {
  let shift = token.substr(0, 1).charCodeAt(0);
  const A = token.substr(1).split("");
  let outStr = [];
  let arr = strIn.split("");
  for (let i in arr) {
    let value = arr[i];
    outStr.push(A[value.charCodeAt(0) - shift]);
  }
  return outStr.join("");
}

const token = 'C7AHISDN2WGQMZFCPL9TRV3OJ6KBEUX851Y4';
const strIn = ']OcfRN'; // input, 6 chars
console.log(dec(strIn, token)); // output, 5 chars