Proof of concept arraybuffer to base64 and back in k6 based on https://gist.github.com/jonleighton/958841

test.js

function decodeArrayBuffer() {
	console.log("------decodeEncodeFails()--------------");
	const b64data = 'YAmMzu2PW2vfpUj22Dli4sk8I5muWlietj/gJR46gUIuWnswfaaT6XneRmP7oS34tUokHKAyL3jalq5cw7FFeA==';

	var a = Base64Binary.decodeArrayBuffer(b64data)
	console.log(a.byteLength);
	console.log(a.length);
	let enc = base64ArrayBuffer(a);
	console.log("before   = " + b64data);
	console.log("after   = " + enc);
	console.log("encode/decode " + (enc == b64data ? "PASSED" : "FAILED"));  // FAIL
}

export default function() {
	decodeArrayBuffer();
}

/*
Copyright (c) 2011, Daniel Guerrero
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
 * Redistributions of source code must retain the above copyright
	  notice, this list of conditions and the following disclaimer.
 * Redistributions in binary form must reproduce the above copyright
	  notice, this list of conditions and the following disclaimer in the
	  documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL DANIEL GUERRERO BE LIABLE FOR ANY
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

/**
 * Uses the new array typed in javascript to binary base64 encode/decode
 * at the moment just decodes a binary base64 encoded
 * into either an ArrayBuffer (decodeArrayBuffer)
 * or into an Uint8Array (decode)
 * 
 * References:
 * https://developer.mozilla.org/en/JavaScript_typed_arrays/ArrayBuffer
 * https://developer.mozilla.org/en/JavaScript_typed_arrays/Uint8Array
 */

var Base64Binary = {
	_keyStr : "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",

	/* will return a  Uint8Array type */
	decodeArrayBuffer: function(input) {
		return this.decode(input);
	},

	removePaddingChars: function(input){
		var lkey = this._keyStr.indexOf(input.charAt(input.length - 1));
		if(lkey == 64){
			return input.substring(0,input.length - 1);
		}
		return input;
	},

	decode: function (input, arrayBuffer) {
		//get last chars to see if are valid
		var n = 0; // added
		var bytes = parseInt((input.length / 4) * 3, 10);

		var uarray;
		var chr1, chr2, chr3;
		var enc1, enc2, enc3, enc4;
		var i = 0;
		var j = 0;

		if (arrayBuffer)
			uarray = new Uint8Array(arrayBuffer);
		else
			uarray = new Uint8Array(bytes);

		input = input.replace(/[^A-Za-z0-9\+\/\=]/g, "");

		for (i=0; i<bytes; i+=3) {	
			//get the 3 octects in 4 ascii chars
			enc1 = this._keyStr.indexOf(input.charAt(j++));
			enc2 = this._keyStr.indexOf(input.charAt(j++));
			enc3 = this._keyStr.indexOf(input.charAt(j++));
			enc4 = this._keyStr.indexOf(input.charAt(j++));

			chr1 = (enc1 << 2) | (enc2 >> 4);
			chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
			chr3 = ((enc3 & 3) << 6) | enc4;

			uarray[i] = chr1;			
			if (enc4 != 64) uarray[i+1] = chr2;
			else n++; //added
			if (enc4 != 64) uarray[i+2] = chr3;
			else n++; // added
		}

		return uarray.buffer.slice(0,-n); // added
	}
}




// 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.length
	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
}