ArrayBuffer to base64 encoded string

I need an efficient (read native) way to convert an ArrayBuffer to a base64 string which needs to be used on a multipart post.

function _arrayBufferToBase64( 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 );

but, non-native implementations are faster e.g. see

This works fine for me:

var base64String = btoa(String.fromCharCode.apply(null, new Uint8Array(arrayBuffer)));

In ES6, the syntax is a little simpler:

let base64String = btoa(String.fromCharCode( Uint8Array(arrayBuffer)));

As pointed out in the comments, this method may result in a runtime error in some browsers when the ArrayBuffer is large. The exact size limit is implementation dependent in any case.

For those who like it short, here's an other one using Array.reduce which will not cause stack overflow:

var base64 = btoa(
  new Uint8Array(arrayBuffer)
    .reduce((data, byte) => data + String.fromCharCode(byte), '')

There is another asynchronous way use Blob and FileReader.

I didn't test the performance. But it is a different way of thinking.

function arrayBufferToBase64( buffer, callback ) {
    var blob = new Blob([buffer],{type:'application/octet-binary'});
    var reader = new FileReader();
    reader.onload = function(evt){
        var dataurl =;

var buf = new Uint8Array([11,22,33]);
arrayBufferToBase64(buf, console.log.bind(console)); //"CxYh"

I used this and works for me.

function arrayBufferToBase64( 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 );

function base64ToArrayBuffer(base64) {
    var binary_string =  window.atob(base64);
    var len = binary_string.length;
    var bytes = new Uint8Array( len );
    for (var i = 0; i < len; i++)        {
        bytes[i] = binary_string.charCodeAt(i);
    return bytes.buffer;

My recommendation for this is to NOT use native btoa strategies—as they don't correctly encode all ArrayBuffer's…

rewrite the DOMs atob() and btoa()

Since DOMStrings are 16-bit-encoded strings, in most browsers calling window.btoa on a Unicode string will cause a Character Out Of Range exception if a character exceeds the range of a 8-bit ASCII-encoded character.

While I have never encountered this exact error, I have found that many of the ArrayBuffer's I have tried to encode have encoded incorrectly.

I would either use MDN recommendation or gist.

var blob = new Blob([arrayBuffer])

var reader = new FileReader();
reader.onload = function(event){
   var base64 =


Below are 2 simple functions for converting Uint8Array to Base64 String and back again

arrayToBase64String(a) {
    return btoa(String.fromCharCode(...a));

base64StringToArray(s) {
    let asciiString = atob(s);
    return new Uint8Array([...asciiString].map(char => char.charCodeAt(0)));

You can derive a normal array from the ArrayBuffer by using Array.prototype.slice. Use a function like to convert bytes in to characters and join them together to forma string.

function arrayBufferToBase64(ab){

    var dView = new Uint8Array(ab);   //Get a byte view        

    var arr =; //Create a normal array        

    var arr1 ={        
      return String.fromCharCode(item);    //Convert

    return window.btoa(arr1.join(''));   //Form a string


This method is faster since there are no string concatenations running in it.

The OP did not specify the Running Enviroment but if you are using Node.JS there is a very simple way to do such thing.

Accordig with the official Node.JS docs

// This step is only necessary if you don't already have a Buffer Object
const buffer = Buffer.from(yourArrayBuffer);

const base64String = buffer.toString('base64');

Also, If you are running under Angular for example, the Buffer Class will also be made available in a Browser Environment.

By my side, using Chrome navigator, I had to use DataView() to read an arrayBuffer

function _arrayBufferToBase64( tabU8A ) {
var binary = '';
let lecteur_de_donnees = new DataView(tabU8A);
var len = lecteur_de_donnees.byteLength;
var chaine = '';
var pos1;
for (var i = 0; i < len; i++) {
    binary += String.fromCharCode( lecteur_de_donnees.getUint8( i ) );
chaine = window.btoa( binary )
return chaine;}

function _arrayBufferToBase64(uarr) {
    var strings = [], chunksize = 0xffff;
    var len = uarr.length;

    for (var i = 0; i * chunksize < len; i++){
        strings.push(String.fromCharCode.apply(null, uarr.subarray(i * chunksize, (i + 1) * chunksize)));

    return strings.join("");

This is better, if you use JSZip for unpack archive from string