Byte to Base64 implementation in JavaScript

base64ToByte.js

const base64ToByte = (base64) => {
  const key = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
  let newBytes = []
  let currentChar = 0
  let currentByte = 0
  for (let i=0; i<base64.length; i++) {     // Go over four 6-bit base64 chars to decode into three 8-bit bytes
    currentChar = key.indexOf(base64[i])
    if (i%4===0) { // First base64 char
      currentByte = (currentChar << 2)      // Get the 6-bits from first base64 char
    }
    if (i%4===1) { // Second base64 char
      currentByte += (currentChar >> 6)     // Concat the first 2-bits from second base64 char
      newBytes.push(currentByte)            // Push the first byte
      currentByte = (currentChar & 15) << 4 // Erase bits before last 4-bits, add last 4-bits for second byte
    }
    if (i%4===2) { // Third base64 char
      currentByte += (currentChar >> 2)     // Concat first 4-bits from third base64 char for second byte
      newBytes.push(currentByte)            // Push second byte
      currentByte = (currentChar & 3) << 6  // Erase bits before last 2-bits, add last 2-bits for third byte
    }
    if (i%4===3) { // Fourth base64 char
      currentByte += (currentChar)          // Concat fourth base64 char for third byte
      newBytes.push(currentByte)            // Push third byte
    }
  }
  if (newBytes[newBytes.length-1] === -1) { // Remove single padding from the end (=)
    newBytes.pop()
  }
  if (newBytes[newBytes.length-2] === -1) { // Remove double padding from the end (==)
    newBytes.pop()
    newBytes.pop()
  }
  return new Uint8Array(newBytes)
}
// base64ToByte('aGVsbG8=')
// hell   => 104 101 108 108         => aGVsbA==
// hello  => 104 101 108 108 111     => aGVsbG8=
// hellos => 104 101 108 108 111 115 => aGVsbG9z

byteToBase64.js

const byteToBase64 = (byte) => {
    const key = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
    let bytes = new Uint8Array(byte)
    let newBase64 = ''
    let currentChar = 0
    for (let i=0; i<bytes.length; i++) {   // Go over three 8-bit bytes to encode four base64 6-bit chars
        if (i%3===0) { // First Byte
            currentChar = (bytes[i] >> 2)      // First 6-bits for first base64 char
            newBase64 += key[currentChar]      // Add the first base64 char to the string
            currentChar = (bytes[i] << 4) & 63 // Erase first 6-bits, add first 2 bits for second base64 char
        }
        if (i%3===1) { // Second Byte
            currentChar += (bytes[i] >> 4)     // Concat first 4-bits from second byte for second base64 char
            newBase64 += key[currentChar]      // Add the second base64 char to the string
            currentChar = (bytes[i] << 2) & 63 // Add two zeros, add 4-bits from second half of second byte
        }
        if (i%3===2) { // Third Byte
            currentChar += (bytes[i] >> 6)     // Concat first 2-bits of third byte for the third base64 char
            newBase64 += key[currentChar]      // Add the third base64 char to the string
            currentChar = bytes[i] & 63        // Add last 6-bits from third byte for the fourth base64 char
            newBase64 += key[currentChar]      // Add the fourth base64 char to the string
        }
    }
    if (bytes.length%3===1) { // Pad for two missing bytes
        newBase64 += `${key[currentChar]}==`
    }
    if (bytes.length%3===2) { // Pad one missing byte
        newBase64 += `${key[currentChar]}=`
    }
    return newBase64
}
// byteToBase64([104,101,108,108,111]) 
// => 'aGVsbG8='

Thank you, your code has helped me with one of my projects! 😉