Convert base64 string to ArrayBuffer

I need to convert a base64 encode string into an ArrayBuffer. The base64 strings are user input, they will be copy and pasted from an email, so they're not there when the page is loaded. I would like to do this in javascript without making an ajax call to the server if possible.

I found those links interesting, but they didt'n help me:

ArrayBuffer to base64 encoded string

this is about the opposite conversion, from ArrayBuffer to base64, not the other way round

this looks good but i can't figure out how to use the code.

Is there an easy (maybe native) way to do the conversion? thanks


Answer's answer does not work because of unicode problem in javascript -

I ended up using the function given on Daniel Guerrero's blog:

Function is listed on github link:

Use these lines

var uintArray = Base64Binary.decode(base64_string);  
var byteArray = Base64Binary.decodeArrayBuffer(base64_string); 

Just found base64-arraybuffer, a small npm package with incredibly high usage, 5M downloads last month (2017-08).

For anyone looking for something of a best standard solution, this may be it.


Javascript is a fine development environment so it seems odd than it doesn't provide a solution to this small problem. The solutions offered elsewhere on this page are potentially slow. Here is my solution. It employs the inbuilt functionality that decodes base64 image and sound data urls.

var req = new XMLHttpRequest;'GET', "data:application/octet;base64," + base64Data);
req.responseType = 'arraybuffer';
req.onload = function fileLoaded(e)
   var byteArray = new Int8Array(;
   // var shortArray = new Int16Array(;
   // var unsignedShortArray = new Int16Array(;
   // etc.

The send request fails if the base 65 string is badly formed.

The mime type (application/octet) is probably unnecessary.

Tested in chrome. Should work in other browsers.


Async solution, it's better when the data is big:

// base64 to buffer
function base64ToBufferAsync(base64) {
  var dataUrl = "data:application/octet-binary;base64," + base64;

    .then(res => res.arrayBuffer())
    .then(buffer => {
      console.log("base64 to buffer: " + new Uint8Array(buffer));

// buffer to base64
function bufferToBase64Async( buffer ) {
    var blob = new Blob([buffer], {type:'application/octet-binary'});    
    console.log("buffer to blob:" + blob)

    var fileReader = new FileReader();
    fileReader.onload = function() {
      var dataUrl = fileReader.result;
      console.log("blob to dataUrl: " + dataUrl);

      var base64 = dataUrl.substr(dataUrl.indexOf(',')+1)      
      console.log("dataUrl to base64: " + base64);
const str = "dGhpcyBpcyBiYXNlNjQgc3RyaW5n"
const encoded = new TextEncoder().encode(str) // is Uint8Array
const buf = encoded.buffer // is ArrayBuffer

Try this:

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;

Using TypedArray.from:

Uint8Array.from(atob(base64_string), c => c.charCodeAt(0))

Performance to be compared with the for loop version of answer.


For Node.js users:

const myBuffer = Buffer.from(someBase64String, 'base64');

myBuffer will be of type Buffer which is a subclass of Uint8Array. Unfortunately, Uint8Array is NOT an ArrayBuffer as the OP was asking for. But when manipulating an ArrayBuffer I almost always wrap it with Uint8Array or something similar, so it should be close to what's being asked for.


Pure JS - no string middlestep (no atob)

I write following function which convert base64 in direct way (without conversion to string at the middlestep). IDEA

  • get 4 base64 characters chunk
  • find index of each character in base64 alphabet
  • convert index to 6-bit number (binary string)
  • join four 6 bit numbers which gives 24-bit numer (stored as binary string)
  • split 24-bit string to three 8-bit and covert each to number and store them in output array
  • corner case: if input base64 string ends with one/two = char, remove one/two numbers from output array

Below solution allows to process large input base64 strings. Similar function for convert bytes to base64 without btoa is HERE

function base64ToBytesArr(str) {
  const abc = [..."ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"]; // base64 alphabet
  let result = [];

  for(let i=0; i<str.length/4; i++) {
    let chunk = [...str.slice(4*i,4*i+4)]
    let bin => abc.indexOf(x).toString(2).padStart(6,0)).join(''); 
    let bytes = bin.match(/.{1,8}/g).map(x=> +('0b'+x));
    result.push(...bytes.slice(0,3 - (str[4*i+2]=="=") - (str[4*i+3]=="=")));
  return result;

// --------
// --------

let test = "Alice's Adventure in Wonderland.";  

console.log('test string:', test.length, test);
let b64_btoa = btoa(test);
console.log('encoded string:', b64_btoa);

let decodedBytes = base64ToBytesArr(b64_btoa); // decode base64 to array of bytes
console.log('decoded bytes:', JSON.stringify(decodedBytes));
let decodedTest = => String.fromCharCode(b) ).join``;
console.log('Uint8Array', JSON.stringify(new Uint8Array(decodedBytes)));
console.log('decoded string:', decodedTest.length, decodedTest);


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