Run results for: String.fromCharCode & btoa vs base64ArrayBuffer function FIXED ON LARGE ARRAY

Run details:

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.164 Safari/537.36

Browser: Chrome 91

Operating system: Windows

Device Platform: Desktop

Date tested: 3 years ago

Test name	Executions per second
String.fromCharCode & btoa	101340.0 Ops/sec
base64ArrayBuffer function	64331.1 Ops/sec

Script Preparation code:

var bytes = Array.from({length: 1000}, e => 0xff * Math.random() | 0);
var uint16 = new Uint16Array(bytes);
var uint8 = new Uint8Array(bytes);

function base64ArrayBuffer(arrayBuffer) {
  var base64    = '';
  var encodings = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';

  var bytes         = 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;
}

Tests:

String.fromCharCode & btoa
globalThis.btoa(String.fromCharCode.apply(null, uint16));
base64ArrayBuffer function
base64ArrayBuffer(uint8);