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llzes/base64ToByte.js

Last active Feb 11, 2020
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Byte to Base64 implementation in JavaScript
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
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='
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