base64 encode/decode with Uint8Array on browser

base64.js

const table = [];
for (let i = 65; i < 91; i++) table.push(String.fromCharCode(i));
for (let i = 97; i < 123; i++) table.push(String.fromCharCode(i));
for (let i = 0; i < 10; i++) table.push(i.toString(10));
table.push("+");
table.push("/");

const rev_table = table.reduce((obj, c, i) => {
  obj[c] = i;
  return obj;
}, {});

function encode(bytes) {
  if (bytes instanceof Array) { 
     bytes = Uint8Array.from(bytes);
  } else if(!(bytes instanceof Uint8Array || bytes instanceof Uint8ClampedArray)) {
     bytes = new TextEncoder().encode(bytes.toString());
  } 
  
  let base64 = "";
  const len = bytes.byteLength;
  for (let i = 0; i < len; i += 3) {
    if (len === i + 1) {  // last 1 byte
      const a = (bytes[i] & 0xfc) >> 2;
      const b = ((bytes[i] & 0x03) << 4);
      base64 += table[a];
      base64 += table[b];
      base64 += "==";
    } else if (len === i + 2) { // last 2 bytes
      const a = (bytes[i] & 0xfc) >> 2;
      const b = ((bytes[i] & 0x03) << 4) | ((bytes[i+1] & 0xf0) >> 4);
      const c = ((bytes[i+1] & 0x0f) << 2);
      base64 += table[a];
      base64 += table[b];
      base64 += table[c];
      base64 += "=";
    } else {
      const a = (bytes[i] & 0xfc) >> 2;
      const b = ((bytes[i] & 0x03) << 4) | ((bytes[i+1] & 0xf0) >> 4);
      const c = ((bytes[i+1] & 0x0f) << 2) | ((bytes[i+2] & 0xc0) >> 6);
      const d = bytes[i+2] & 0x3f;
      base64 += table[a];
      base64 += table[b];
      base64 += table[c];
      base64 += table[d];
    }
  }
  return base64;
}

function decode(base64str) {
  const len = base64str.length;
  //if (len % 4 !== 0) {
  //  base64str = base64str.padEnd(len + 4 - (len % 4), "=")
  //}
  
  const bytes = [];
  for (let i = 0; i < len; i += 4) {
    const a = rev_table[base64str[i  ]];
    const b = rev_table[base64str[i+1]];
    const c = rev_table[base64str[i+2]];
    const d = rev_table[base64str[i+3]];
    bytes.push((a << 2 & 0xfc) | (b >> 4 & 0x03));
    bytes.push((b << 4 & 0xf0) | (c >> 2 & 0x0f));
    bytes.push((c << 6 & 0xc0) | d & 0x3f);
  }
  return bytes;
}

function decodeAsArrayBuffer(base64str) {
  return Uint8Array.from(decode(base64str));
}

function decodeAsString(base64str) {
  return new TextDecoder().decode(decodeAsArrayBuffer(base64str));
}