Skip to content

Instantly share code, notes, and snippets.

@jonleighton
Last active November 2, 2024 06:08
Show Gist options
  • Save jonleighton/958841 to your computer and use it in GitHub Desktop.
Save jonleighton/958841 to your computer and use it in GitHub Desktop.
Encode an ArrayBuffer as a base64 string
// Converts an ArrayBuffer directly to base64, without any intermediate 'convert to string then
// use window.btoa' step. According to my tests, this appears to be a faster approach:
// http://jsperf.com/encoding-xhr-image-data/5
/*
MIT LICENSE
Copyright 2011 Jon Leighton
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
function base64ArrayBuffer(arrayBuffer) {
var base64 = ''
var encodings = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
var bytes = new Uint8Array(arrayBuffer)
var byteLength = bytes.byteLength
var byteRemainder = byteLength % 3
var mainLength = byteLength - byteRemainder
var a, b, c, d
var chunk
// Main loop deals with bytes in chunks of 3
for (var i = 0; i < mainLength; i = i + 3) {
// Combine the three bytes into a single integer
chunk = (bytes[i] << 16) | (bytes[i + 1] << 8) | bytes[i + 2]
// Use bitmasks to extract 6-bit segments from the triplet
a = (chunk & 16515072) >> 18 // 16515072 = (2^6 - 1) << 18
b = (chunk & 258048) >> 12 // 258048 = (2^6 - 1) << 12
c = (chunk & 4032) >> 6 // 4032 = (2^6 - 1) << 6
d = chunk & 63 // 63 = 2^6 - 1
// Convert the raw binary segments to the appropriate ASCII encoding
base64 += encodings[a] + encodings[b] + encodings[c] + encodings[d]
}
// Deal with the remaining bytes and padding
if (byteRemainder == 1) {
chunk = bytes[mainLength]
a = (chunk & 252) >> 2 // 252 = (2^6 - 1) << 2
// Set the 4 least significant bits to zero
b = (chunk & 3) << 4 // 3 = 2^2 - 1
base64 += encodings[a] + encodings[b] + '=='
} else if (byteRemainder == 2) {
chunk = (bytes[mainLength] << 8) | bytes[mainLength + 1]
a = (chunk & 64512) >> 10 // 64512 = (2^6 - 1) << 10
b = (chunk & 1008) >> 4 // 1008 = (2^6 - 1) << 4
// Set the 2 least significant bits to zero
c = (chunk & 15) << 2 // 15 = 2^4 - 1
base64 += encodings[a] + encodings[b] + encodings[c] + '='
}
return base64
}
@TFrascaroli
Copy link

@peterbajwa Base64 is base64, no matter who produces it or who consumes it. That's the beauty of standards 😉

@YATACOBLAS
Copy link

thank you !!! @jonleighton

@Github743
Copy link

When I am uploading a 10 MB file some how it is not giving any conversion. Here is what I am working on
https://jsfiddle.net/DorababuMeka/7g9yLrs3/29/

@yongky-utama
Copy link

yongky-utama commented Nov 10, 2021

@hookenz
I know it's been a long time, but as I tried to figure out why it didn't work, I found that in your loop, buffer[i] is always undefined.
It turned out that you just need to convert it to Uint8Array first.

function uint8ToBase64( buffer ) {
    var binary = '';
    var bytes = new Uint8Array(buffer);
    var len = bytes.byteLength;
    for (var i = 0; i < len; i++) {
        binary += String.fromCharCode(bytes[i]);
    }
    return window.btoa( binary );
}

Which is the same as the top answer in this stack overflow topic: ( https://stackoverflow.com/questions/9267899/arraybuffer-to-base64-encoded-string )

It's correct as far as I can tell, as modern browsers allows '\x00' characters in JS Strings.

@EmanH
Copy link

EmanH commented Feb 25, 2022

Updated benchmarks: https://jsben.ch/wnaZC

@LittleSaya
Copy link

I wrote a url-safe version with a corresponding decoding function in typescript: base64ArrayBuffer.ts

@makarworld
Copy link

I wrote this code on python with decode function: base64u

Also it published on PyPi

pip install base64u

@Vanilagy
Copy link

@LittleSaya Thanks man, your code's the bomb and works like a charm. I validated it with a fuzztest and it produces correct results!

@pirumu
Copy link

pirumu commented May 21, 2024

I wrote a url-safe version with a corresponding decoding function in typescript: base64ArrayBuffer.ts

@LittleSaya You’ve saved my life <3!

@vijtheveg
Copy link

// Use a lookup table to find the index.
var lookup = new Uint8Array(256);
for (var i = 0; i < chars.length; i++) {
lookup[chars.charCodeAt(i)] = i;
}

Shouldn't the loop go to lookup.length?

@vijtheveg
Copy link

The problem I am seeing in all the encoder implementations above is that they seem to be creating a string for each byte that this being encoded, and doing a string concatenation for each byte. Am I missing something? Both string creation and string concatenation are very expensive when done in large numbers.

During encoding, isn't it better to get character codes and join them in bulk, using String.fromCharCode(...charCodes)?

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment