Encode an ArrayBuffer as a base64 string

base64ArrayBuffer.js

// 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
}

This is not work in EDGE and Internet Explorer 11. Please, help me :'(

@hookenz - I know this is quite a long way down the road (almost a year after your last coment), but I don't think you see what's going on here and I fear that other people may fall into the same trap. You see, OP is giving us a function to convert ANY arbitrary ArrayBuffer to a base64 string in javascript. Your solution works great aswell, except that it ONLY works when the underlying array of bytes comes from a UTF-8 encoded string, meaning that it can not be used to convert all byte arrays, only those that don't have "invalid" bytes in it, namely the infamous '\0' byte. So it doesn't really matter if it's faster if it doesn't work for all intended cases.

In case anyone is wondering why this is so, it's because of the way String.fromCharCode() works.
For a really small and homemade test, you can do this:

• Assume hookenzBase64 is your function
• Assume jonleightonBase64 is OP function

var b = new Uint16Array(25);
b.set([1,2,3,4,5], 3);
hookenzBase64(b.buffer); //[WRONG], gives "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA="
jonleightonBase64(b.buffer); //[RIGHT] gives "AAAAAAAAAQACAAMABAAFAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA="

I hope this clarifies things for anyone else looking into these two functions ;)

@proton5000 - Tested on Edge/15.15063 and it works for me (with the really barebones test that I posted just now).
Also, it should work, according to CanIUse

I'd like to validate the output string by rendering the image again. What do I need to add to the front to set it as the src attribute of an img tag? I tried this, but just got a broken image. (Failed to load resource: net::ERR_FILE_NOT_FOUND)

<img src="data:image/png;base64,[function result]">

Thanks.

alternative:

  //get ArrayBuffer from somewhere, for example:
  //FileReader's readAsArrayBuffer ".result"
  //and convert them to a BASE64-text.

function simpler_way(buffer){
  var tmp;

  tmp = (new TextDecoder("utf-8")).decode(buffer); //to UTF-8 text.
  tmp = unescape(encodeURIComponent(tmp));         //to binary-string.
  tmp = btoa(tmp);                                 //BASE64.
  return tmp;

//you can stop here.
//but if you need to return an ArrayBuffer of the BASE64 result,
//for example to be passed from a Worker back to a client, 
//by using 'postMessage' + Transferable object (much faster see: https://developer.mozilla.org/en-US/docs/Web/API/Transferable),
//uncomment lines below:
  
//tmp = (new TextEncoder("utf-8")).encode(tmp);    //to Uint8Array.
//tmp = tmp.buffer;                                //to ArrayBuffer.
//return tmp;
}

_{used in this repository,
as a Worker-FileReader example
(a no-upload file to Base64 converter )}

Hi, this library exist in bower?

Thank you! I finally came across this gist after a few hours of banging my head against a wall. I was fetching a PNG as an ArrayBuffer and needed to convert to a base64 encoding so I could automatically save the png.

I was using node's Buffer to encode values and then decode in python using base64 package and produced false results on python side.
Using this gist I was able to successfully decode values in python.
You saved my day. Many many Thanks for sharing.

THANKS YOU SOOO MUCH

based on https://developers.google.com/web/updates/2012/06/How-to-convert-ArrayBuffer-to-and-from-String,
a more easier way:

function bufferToBase64(buffer) {
     const binary = String.fromCharCode.apply(null, buffer);
     return window.btoa(binary);
}

@Teamop this way wont work for ArrayBuffer larger than 30k bytes.

I ended up using this

(function(){
  "use strict";

  var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

  // 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;
  }

  exports.encode = function(arraybuffer) {
    var bytes = new Uint8Array(arraybuffer),
    i, len = bytes.length, base64 = "";

    for (i = 0; i < len; i+=3) {
      base64 += chars[bytes[i] >> 2];
      base64 += chars[((bytes[i] & 3) << 4) | (bytes[i + 1] >> 4)];
      base64 += chars[((bytes[i + 1] & 15) << 2) | (bytes[i + 2] >> 6)];
      base64 += chars[bytes[i + 2] & 63];
    }

    if ((len % 3) === 2) {
      base64 = base64.substring(0, base64.length - 1) + "=";
    } else if (len % 3 === 1) {
      base64 = base64.substring(0, base64.length - 2) + "==";
    }

    return base64;
  };

  exports.decode =  function(base64) {
    var bufferLength = base64.length * 0.75,
    len = base64.length, i, p = 0,
    encoded1, encoded2, encoded3, encoded4;

    if (base64[base64.length - 1] === "=") {
      bufferLength--;
      if (base64[base64.length - 2] === "=") {
        bufferLength--;
      }
    }

    var arraybuffer = new ArrayBuffer(bufferLength),
    bytes = new Uint8Array(arraybuffer);

    for (i = 0; i < len; i+=4) {
      encoded1 = lookup[base64.charCodeAt(i)];
      encoded2 = lookup[base64.charCodeAt(i+1)];
      encoded3 = lookup[base64.charCodeAt(i+2)];
      encoded4 = lookup[base64.charCodeAt(i+3)];

      bytes[p++] = (encoded1 << 2) | (encoded2 >> 4);
      bytes[p++] = ((encoded2 & 15) << 4) | (encoded3 >> 2);
      bytes[p++] = ((encoded3 & 3) << 6) | (encoded4 & 63);
    }

    return arraybuffer;
  };
})();

function bufferToBase64(buffer) {
     const binary = String.fromCharCode.apply(null, buffer);
     return window.btoa(binary);
}

@Teamop this way wont work for ArrayBuffer larger than 30k bytes.

While this thread was about "Converts an ArrayBuffer directly to base64, without any intermediate 'convert to string then use window.btoa' step" so I'm going a bit off topic, but there is a way to avoid the "larger than 30k bytes" issues using reduce.

bufferToBase64(buffer) {
    return btoa(new Uint8Array(buffer).reduce((data, byte)=> {
      return data + String.fromCharCode(byte);
    }, ''));
  }

Further example using this with angular7 for image source (say you're storing images in a database)

  bufferToBase64ImageSource(buffer) {
    const base64String = btoa(new Uint8Array(buffer).reduce((data, byte)=> {
      return data + String.fromCharCode(byte);
    }, ''));
    return this.domSanitzer.bypassSecurityTrustUrl('data:image/jpg;base64, ' + base64String);
  }

@Teamop this way wont work for ArrayBuffer larger than 30k bytes.

Can you elaborate why it wouldn't work? I've been using it successfully in both Firefox and Chrome?

I've made a benchmark: https://jsbench.me/vjk9nczxst/1

The code below shows stable and fast result among browsers.

function bufferToBase64(buffer) {
     const binary = String.fromCharCode.apply(null, buffer);
     return window.btoa(binary);
}

However, original approach is a little bit faster in Safari

Warning: Using @spalt08's method with the recursion will fail with a RangeError: Maximum call stack size exceeded error for large buffers.

Better to use a good ol' for loop, like @hookenz. Sometimes functional programming is terribly inefficient.

@odbol
there is no recursion here. If your payload is big, String.fromCharCode.apply will overflow execution stack. You can use for loop for converting buffer to binary string in these cases

@TFrascaroli - thanks for the feedback in 2016! As I recall I needed the utmost performance when converting a UTF-8 encoded string. It was something to do with encoding images for something like an MJPEG stream so performance was important and 40ms made a difference. Anyway, thanks for pointing out the differences.

@hookenz To be perfectly frank, I don't even remember what I used this for. But I remember that I needed a way to convert binary data into base64, not just binary representation of a string, but actual binary data with a bunch of '\0's in it. Anyway, if I helped in any way I'm glad I could do so 👍

I found that converting to a string (with String.fromCharCode) and then base64 encoding (with btoa) is faster, it may have changed
https://www.measurethat.net/Benchmarks/Show/10469/0/stringfromcharcode-btoa-vs-base64arraybuffer-function

edit: I just noticed and fixed a flaw in the test above, a new arraybuffer is being created in the function, and the whole point of this gist is avoiding copying. I fixed it here https://www.measurethat.net/Benchmarks/Show/10470/0/stringfromcharcode-btoa-vs-base64arraybuffer-function-f
turns out that base64 encoding an arraybuffer to string is faster than creating a DOMString then base64 encoding to a DOMstring.

edit2: with a big arraybuffer the performance tanks and performance of native String.fromCharCode stays more consistent
https://www.measurethat.net/Benchmarks/Show/10471/0/stringfromcharcode-btoa-vs-base64arraybuffer-function-f

Can anyone please help me know, is this base64 conversion specific to Javascript or is equivalent to other languages Base64 conversion methods. eg. Ruby has below code to convert the byte array to base64 string.
array.pack('C*').force_encoding('UTF-8')

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

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

thank you !!! @jonleighton

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/

@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.

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

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

I wrote this code on python with decode function: base64u

Also it published on PyPi

pip install base64u