How to convert uint8 Array to base64 Encoded String?

Question

I got a webSocket comunication, I recieve base64 encoded string, convert it to uint8 and work on it, but now I need to send back, I got the uint8 array, and need to convert it to base64 string, so I can send it. How can I make this convertion?

Answer 1

If your data may contain multi-byte sequences (not a plain ASCII sequence) and your browser has TextDecoder, then you should use that to decode your data (specify the required encoding for the TextDecoder):

var u8 = new Uint8Array([65, 66, 67, 68]);
var decoder = new TextDecoder('utf8');
var b64encoded = btoa(decoder.decode(u8));

If you need to support browsers that do not have TextDecoder (currently just IE and Edge), then the best option is to use a TextDecoder polyfill.

If your data contains plain ASCII (not multibyte Unicode/UTF-8) then there is a simple alternative using String.fromCharCode that should be fairly universally supported:

var ascii = new Uint8Array([65, 66, 67, 68]);
var b64encoded = btoa(String.fromCharCode.apply(null, ascii));

And to decode the base64 string back to a Uint8Array:

var u8_2 = new Uint8Array(atob(b64encoded).split("").map(function(c) {
    return c.charCodeAt(0); }));

If you have very large array buffers then the apply may fail with Maximum call stack size exceeded and you may need to chunk the buffer (based on the one posted by @RohitSengar). Again, note that this is only correct if your buffer only contains non-multibyte ASCII characters:

function Uint8ToString(u8a){
  var CHUNK_SZ = 0x8000;
  var c = [];
  for (var i=0; i < u8a.length; i+=CHUNK_SZ) {
    c.push(String.fromCharCode.apply(null, u8a.subarray(i, i+CHUNK_SZ)));
  }
  return c.join("");
}
// Usage
var u8 = new Uint8Array([65, 66, 67, 68]);
var b64encoded = btoa(Uint8ToString(u8));

Answer 2

If you are using Node.js then you can use this code to convert Uint8Array to base64

var u8 = new Uint8Array([65, 66, 67, 68]);
var b64 = Buffer.from(u8).toString('base64');

Answer 3

Native browser solution (fast!)

To base64-encode a Uint8Array with arbitrary data (not necessarily UTF-8) using native browser functionality:

// note: `buffer` arg can be an ArrayBuffer or a Uint8Array
async function bufferToBase64(buffer) {
  // use a FileReader to generate a base64 data URI:
  const base64url = await new Promise(r => {
    const reader = new FileReader()
    reader.onload = () => r(reader.result)
    reader.readAsDataURL(new Blob([buffer]))
  });
  // remove the `data:...;base64,` part from the start
  return base64url.slice(base64url.indexOf(',') + 1);
}

// example use:
await bufferToBase64(new Uint8Array([1,2,3,100,200]))

Because this is using native browser features, the performance is optimal. It can convert 250 MB per second on my computer (benchmark script), making it about 60x faster than the accepted answer.

Answer 4

All solutions already proposed have severe problems. Some solutions fail to work on large arrays, some provide wrong output, some throw an error on btoa call if an intermediate string contains multibyte characters, some consume more memory than needed.

So I implemented a direct conversion function which just works regardless of the input. It converts about 5 million bytes per second on my machine.

https://gist.github.com/enepomnyaschih/72c423f727d395eeaa09697058238727

/*
MIT License
Copyright (c) 2020 Egor Nepomnyaschih
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.
*/

/*
// This constant can also be computed with the following algorithm:
const base64abc = [],
 A = "A".charCodeAt(0),
 a = "a".charCodeAt(0),
 n = "0".charCodeAt(0);
for (let i = 0; i < 26; ++i) {
 base64abc.push(String.fromCharCode(A + i));
}
for (let i = 0; i < 26; ++i) {
 base64abc.push(String.fromCharCode(a + i));
}
for (let i = 0; i < 10; ++i) {
 base64abc.push(String.fromCharCode(n + i));
}
base64abc.push("+");
base64abc.push("/");
*/
const base64abc = [
 "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
 "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z",
 "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m",
 "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z",
 "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "+", "/"
];

/*
// This constant can also be computed with the following algorithm:
const l = 256, base64codes = new Uint8Array(l);
for (let i = 0; i < l; ++i) {
 base64codes[i] = 255; // invalid character
}
base64abc.forEach((char, index) => {
 base64codes[char.charCodeAt(0)] = index;
});
base64codes["=".charCodeAt(0)] = 0; // ignored anyway, so we just need to prevent an error
*/
const base64codes = [
 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 62, 255, 255, 255, 63,
 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 255, 255, 255, 0, 255, 255,
 255, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 255, 255, 255, 255, 255,
 255, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51
];

function getBase64Code(charCode) {
 if (charCode >= base64codes.length) {
  throw new Error("Unable to parse base64 string.");
 }
 const code = base64codes[charCode];
 if (code === 255) {
  throw new Error("Unable to parse base64 string.");
 }
 return code;
}

export function bytesToBase64(bytes) {
 let result = '', i, l = bytes.length;
 for (i = 2; i < l; i += 3) {
  result += base64abc[bytes[i - 2] >> 2];
  result += base64abc[((bytes[i - 2] & 0x03) << 4) | (bytes[i - 1] >> 4)];
  result += base64abc[((bytes[i - 1] & 0x0F) << 2) | (bytes[i] >> 6)];
  result += base64abc[bytes[i] & 0x3F];
 }
 if (i === l + 1) { // 1 octet yet to write
  result += base64abc[bytes[i - 2] >> 2];
  result += base64abc[(bytes[i - 2] & 0x03) << 4];
  result += "==";
 }
 if (i === l) { // 2 octets yet to write
  result += base64abc[bytes[i - 2] >> 2];
  result += base64abc[((bytes[i - 2] & 0x03) << 4) | (bytes[i - 1] >> 4)];
  result += base64abc[(bytes[i - 1] & 0x0F) << 2];
  result += "=";
 }
 return result;
}

export function base64ToBytes(str) {
 if (str.length % 4 !== 0) {
  throw new Error("Unable to parse base64 string.");
 }
 const index = str.indexOf("=");
 if (index !== -1 && index < str.length - 2) {
  throw new Error("Unable to parse base64 string.");
 }
 let missingOctets = str.endsWith("==") ? 2 : str.endsWith("=") ? 1 : 0,
  n = str.length,
  result = new Uint8Array(3 * (n / 4)),
  buffer;
 for (let i = 0, j = 0; i < n; i += 4, j += 3) {
  buffer =
   getBase64Code(str.charCodeAt(i)) << 18 |
   getBase64Code(str.charCodeAt(i + 1)) << 12 |
   getBase64Code(str.charCodeAt(i + 2)) << 6 |
   getBase64Code(str.charCodeAt(i + 3));
  result[j] = buffer >> 16;
  result[j + 1] = (buffer >> 8) & 0xFF;
  result[j + 2] = buffer & 0xFF;
 }
 return result.subarray(0, result.length - missingOctets);
}

export function base64encode(str, encoder = new TextEncoder()) {
 return bytesToBase64(encoder.encode(str));
}

export function base64decode(str, decoder = new TextDecoder()) {
 return decoder.decode(base64ToBytes(str));
}

Answer 5

Very simple solution and test for JavaScript!

ToBase64 = function (u8) {
    return btoa(String.fromCharCode.apply(null, u8));
}

FromBase64 = function (str) {
    return atob(str).split('').map(function (c) { return c.charCodeAt(0); });
}

var u8 = new Uint8Array(256);
for (var i = 0; i < 256; i++)
    u8[i] = i;

var b64 = ToBase64(u8);
console.debug(b64);
console.debug(FromBase64(b64));

Answer 6

function Uint8ToBase64(u8Arr){
  var CHUNK_SIZE = 0x8000; //arbitrary number
  var index = 0;
  var length = u8Arr.length;
  var result = '';
  var slice;
  while (index < length) {
    slice = u8Arr.subarray(index, Math.min(index + CHUNK_SIZE, length)); 
    result += String.fromCharCode.apply(null, slice);
    index += CHUNK_SIZE;
  }
  return btoa(result);
}

You can use this function if you have a very large Uint8Array. This is for Javascript, can be useful in case of FileReader readAsArrayBuffer.

Answer 7

Pure JS - no string middlestep (no btoa)

In below solution I omit conversion to string. IDEA is following:

• join 3 bytes (3 array elements) and you get 24-bits
• split 24bits to four 6-bit numbers (which take values from 0 to 63)
• use that numbers as index in base64 alphabet
• corner case: when input byte array the length is not divided by 3 then add = or == to result

Solution below works on 3-bytes chunks so it is good for large arrays. Similar solution to convert base64 to binary array (without atob) is HERE

function bytesArrToBase64(arr) {
  const abc = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"; // base64 alphabet
  const bin = n => n.toString(2).padStart(8,0); // convert num to 8-bit binary string
  const l = arr.length
  let result = '';

  for(let i=0; i<=(l-1)/3; i++) {
    let c1 = i*3+1>=l; // case when "=" is on end
    let c2 = i*3+2>=l; // case when "=" is on end
    let chunk = bin(arr[3*i]) + bin(c1? 0:arr[3*i+1]) + bin(c2? 0:arr[3*i+2]);
    let r = chunk.match(/.{1,6}/g).map((x,j)=> j==3&&c2 ? '=' :(j==2&&c1 ? '=':abc[+('0b'+x)]));  
    result += r.join('');
  }

  return result;
}


// ----------
// TEST
// ----------

let test = "Alice's Adventure in Wondeland.";
let testBytes = [...test].map(c=> c.charCodeAt(0) );

console.log('test string:', test);
console.log('bytes:', JSON.stringify(testBytes));
console.log('btoa            ', btoa(test));
console.log('bytesArrToBase64', bytesArrToBase64(testBytes));

Caution!

If you want to convert STRING (not bytes array) be aware that btoa in general will fails on utf8 strings like btoa("") (one character may be encoded by more than one byte). In this case you must first convert such string to bytes in proper way and then use above solution e.g. :

function bytesArrToBase64(arr) {
  const abc = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"; // base64 alphabet
  const bin = n => n.toString(2).padStart(8,0); // convert num to 8-bit binary string
  const l = arr.length
  let result = '';

  for(let i=0; i<=(l-1)/3; i++) {
    let c1 = i*3+1>=l; // case when "=" is on end
    let c2 = i*3+2>=l; // case when "=" is on end
    let chunk = bin(arr[3*i]) + bin(c1? 0:arr[3*i+1]) + bin(c2? 0:arr[3*i+2]);
    let r = chunk.match(/.{1,6}/g).map((x,j)=> j==3&&c2 ? '=' :(j==2&&c1 ? '=':abc[+('0b'+x)]));  
    result += r.join('');
  }

  return result;
}


// ----------
// TEST
// ----------

let test = "";   // base64: 8J+SqQ==
let testBytes = new TextEncoder().encode(test);

console.log('test string      :', test);
console.log('bytes            :', JSON.stringify([...testBytes]));
console.log('bytesArrToBase64 :', bytesArrToBase64(testBytes));


try {
  console.log('test btoa :', btoa(test));
} catch (e) {
  console.error('btoa fails during conversion!', e.message)
}

Snippets tested 2022-08-04 on: chrome 103.0.5060.134 (arm64), safari 15.2, firefox 103.0.1 (64 bit), edge 103.0.1264.77 (arm64), and node-js v12.16.1

Answer 8

Use the following to convert uint8 array to base64 encoded string

function arrayBufferToBase64(buffer) {
            var binary = '';
            var bytes = [].slice.call(new Uint8Array(buffer));
            bytes.forEach((b) => binary += String.fromCharCode(b));
            return window.btoa(binary);
        };

Answer 9

MDN's docs cover btoa well.

Because you already have binary data, you can convert your Uint8Array into an ASCII string and invoke btoa on that string.

function encodeBase64Bytes(bytes: Uint8Array): string {
  return btoa(
    bytes.reduce((acc, current) => acc + String.fromCharCode(current), "")
  );
}

Complexity with btoa arises when you need to encode arbitrary JS strings, which may occupy more than a single byte, such as "". To handle arbitrary JS strings (which are UTF-16), you must first convert the string to a single byte representation. This is not applicable for this use case because you already have binary data.

The linked MDN documentation covers what that conversion looks like for encoding (and the reciprocal steps for decoding).

Answer 10

In the browser you can do:

Uint8Array --> Base64

btoa(String.fromCharCode.apply(null,new Uint8Array([1,2,3,255])))

Base64 --> Uint8Array

new Uint8Array([...atob('AQID/w==')].map(c=>c.charCodeAt()))

Answer 11

Since btoa only works with strings, we can stringify the Uint8Array with String.fromCharCode:

const toBase64 = uInt8Array => btoa(String.fromCharCode(...uInt8Array));

Answer 12

Here's a solution that doesn't use the "splat operator":

function uint8ArrayFromBase64(s) {
  // 1. Call atob()
  var b = atob(s);
  // 2. Construct Uint8Array from String
  return Uint8Array.from({
    [Symbol.iterator]() {
      var i = 0, end = b.length,
          b_at = b.charCodeAt.bind(b);
      return ({
        next() {
          if (i > end) return {done: true};
          return {value: b_at(i++)};
        }
      });
    }
  });
}

function uint8ArrayToBase64(a) {
  // 1. Preprocess Uint8Array into String
  // (TODO: fix RAM usage from intermediate array creation)
  var a_s = Array.prototype.map.call(a, c => String.fromCharCode(c)).join(String());
  // 2. Call btoa()
  return btoa(a_s);
}

Demo:

<form action="javascript:" onsubmit="(({target:form,submitter:{value:action}})=>{eval(action)(form)})(event)">
<input name="b64" value="AAAAB3NzaC1yc2E=">
<button type="submit" value="({b64:{value:s},u8a:e})=>{e.value=`[${uint8ArrayFromBase64(s)}]`;}">Convert to Uint8Array</button>
<br />
<input name="u8a" value="">
<button type="submit" value="({u8a:{value:x},b64:e})=>{e.value=(uint8ArrayToBase64(x.replace(/(?:^\[|\]$)/g, '').split(',')));}">Convert to Base64</button>
</form>

Answer 13

Here is a JS Function to this:

This function is needed because Chrome doesn't accept a base64 encoded string as value for applicationServerKey in pushManager.subscribe yet https://bugs.chromium.org/p/chromium/issues/detail?id=802280

function urlBase64ToUint8Array(base64String) {
  var padding = '='.repeat((4 - base64String.length % 4) % 4);
  var base64 = (base64String + padding)
    .replace(/\-/g, '+')
    .replace(/_/g, '/');

  var rawData = window.atob(base64);
  var outputArray = new Uint8Array(rawData.length);

  for (var i = 0; i < rawData.length; ++i) {
    outputArray[i] = rawData.charCodeAt(i);
  }
  return outputArray;
}

Answer 14

If all you want is a JS implementation of a base64-encoder, so that you can send data back, you can try the btoa function.

b64enc = btoa(uint);

A couple of quick notes on btoa - it's non-standard, so browsers aren't forced to support it. However, most browsers do. The big ones, at least. atob is the opposite conversion.

If you need a different implementation, or you find an edge-case where the browser has no idea what you're talking about, searching for a base64 encoder for JS wouldn't be too hard.

I think there are 3 of them hanging around on my company's website, for some reason...

Answer 15

npm install google-closure-library --save

require("google-closure-library");
goog.require('goog.crypt.base64');

var result =goog.crypt.base64.encodeByteArray(Uint8Array.of(1,83,27,99,102,66));
console.log(result);

$node index.js would write AVMbY2Y= to the console.