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