PHP's base64_encode in JavaScript

module.exports = functionbase64_encode (stringToEncode) { // eslint-disable-line camelcase
//  discuss at: https://locutus.io/php/base64_encode/
// original by: Tyler Akins (https://rumkin.com)
// improved by: Bayron Guevara
// improved by: Thunder.m
// improved by: Kevin van Zonneveld (https://kvz.io)
// improved by: Kevin van Zonneveld (https://kvz.io)
// improved by: Rafał Kukawski (https://blog.kukawski.pl)
// bugfixed by: Pellentesque Malesuada
// improved by: Indigo744
//   example 1: base64_encode('Kevin van Zonneveld')
//   returns 1: 'S2V2aW4gdmFuIFpvbm5ldmVsZA=='
//   example 2: base64_encode('a')
//   returns 2: 'YQ=='
//   example 3: base64_encode('✓ à la mode')
//   returns 3: '4pyTIMOgIGxhIG1vZGU='
// encodeUTF8string()
// Internal function to encode properly UTF8 string
// Adapted from Solution #1 at https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding
const encodeUTF8string = function (str) {
// first we use encodeURIComponent to get percent-encoded UTF-8,
// then we convert the percent encodings into raw bytes which
// can be fed into the base64 encoding algorithm.
returnencodeURIComponent(str).replace(/%([0-9A-F]{2})/g,
functiontoSolidBytes (match, p1) {
returnString.fromCharCode('0x' + p1)
      })
  }
if (typeofwindow !== 'undefined') {
if (typeofwindow.btoa !== 'undefined') {
returnwindow.btoa(encodeUTF8string(stringToEncode))
    }
  } else {
returnnew Buffer(stringToEncode).toString('base64')
  }
const b64 = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='
let o1
let o2
let o3
let h1
let h2
let h3
let h4
let bits
let i = 0
let ac = 0
let enc = ''
const tmpArr = []
if (!stringToEncode) {
return stringToEncode
  }
  stringToEncode = encodeUTF8string(stringToEncode)
do {
// pack three octets into four hexets
    o1 = stringToEncode.charCodeAt(i++)
    o2 = stringToEncode.charCodeAt(i++)
    o3 = stringToEncode.charCodeAt(i++)
    bits = o1 << 16 | o2 << 8 | o3
    h1 = bits >> 18 & 0x3f
    h2 = bits >> 12 & 0x3f
    h3 = bits >> 6 & 0x3f
    h4 = bits & 0x3f
// use hexets to index into b64, and append result to encoded string
    tmpArr[ac++] = b64.charAt(h1) + b64.charAt(h2) + b64.charAt(h3) + b64.charAt(h4)
  } while (i < stringToEncode.length)
  enc = tmpArr.join('')
const r = stringToEncode.length % 3
return (r ? enc.slice(0, r - 3) : enc) + '==='.slice(r || 3)
}