TinyChan

function baseNEncode(bytes,alphabet){
  const n=BigInt(alphabet.length);
  let num=BigInt(0);
  //Convert bytes to a BigInt.
  for(let i=0;i<bytes.length;i++){
    num=(num<<8n)+BigInt(bytes[i]);
  }
  let result="";
  if(num===0n){ //Special case for zero value.
    result=alphabet[0];
  }else{
    //Convert the BigInt to base-N string.
    while(num>0n){
      const rem=num%n;
      num/=n;
      result=alphabet[Number(rem)]+result;
    }
  }
  //Preserve leading zero bytes.
  let zeros=0;
  for(let i=0;i<bytes.length&&bytes[i]===0;i++)zeros++;
  //Add leading zeros as the first character in the alphabet.
  result=alphabet[0].repeat(zeros)+result;
  return result;
}
function baseNDecode(str,alphabet){
  const n=BigInt(alphabet.length),
        baseMap=new Map();
  //Create a mapping from characters to their numeric values.
  for(let i=0;i<alphabet.length;i++)
    baseMap.set(alphabet[i], BigInt(i));
  //Count leading characters corresponding to zero bytes.
  let zeros=0;
  for(let i=0;i<str.length&&str[i]===alphabet[0];i++)zeros++;
  //Convert the string back to a BigInt.
  let num=BigInt(0);
  for(let i=zeros;i<str.length;i++)
    num=num*n+baseMap.get(str[i]);
  //Convert the BigInt back to bytes.
  let bytes=[];
  while(num>0n){
    bytes.unshift(Number(num&0xFFn));
    num=num>>8n;
  }
  //Prepend the leading zero bytes.
  const leadingZeros=new Uint8Array(zeros);
  return Uint8Array.from([...leadingZeros,...bytes]);
}

strRange=(a,b)=>Array.from({length:(b.charCodeAt(0)-a.charCodeAt(0))+1},(_,i)=>String.fromCharCode(a.charCodeAt(0)+i)).join("");
base128Alphabet=strRange("0","9")+strRange("A","Z")+strRange("a","z")+strRange("À","ÿ")+"+/"; //0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyzÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ+/

function randomU8array(len){let array=new Uint8Array(len);for(let i=0;i<array.length;i++)array[i]=Math.floor(Math.random()*256);return array;}

function arraysEqual(a,b){
  if(a===b)return true;
  if(a.length!==b.length)return false;
  for(let i=0;i<a.length;i++)
    if(a[i]!==b[i])return false;
  return true;
}

for(let i=0;i<10;i++){
  let randomBytes=randomU8array(10+Math.random()*100),
      encoded=baseNEncode(randomBytes,base128Alphabet),
      decoded=baseNDecode(encoded,base128Alphabet);
  console.log([...randomBytes].map(x=>x.toString(16).padStart(2,"0")));
  console.log(encoded);
  console.log([...decoded].map(x=>x.toString(16).padStart(2,"0")));
  console.log(arraysEqual(randomBytes,decoded));
}