PHP's utf8_encode in JavaScript

module.exports = functionutf8_encode (argString) { // eslint-disable-line camelcase
//  discuss at: https://locutus.io/php/utf8_encode/
// original by: Webtoolkit.info (https://www.webtoolkit.info/)
// improved by: Kevin van Zonneveld (https://kvz.io)
// improved by: sowberry
// improved by: Jack
// improved by: Yves Sucaet
// improved by: kirilloid
// bugfixed by: Onno Marsman (https://twitter.com/onnomarsman)
// bugfixed by: Onno Marsman (https://twitter.com/onnomarsman)
// bugfixed by: Ulrich
// bugfixed by: Rafał Kukawski (https://blog.kukawski.pl)
// bugfixed by: kirilloid
//   example 1: utf8_encode('Kevin van Zonneveld')
//   returns 1: 'Kevin van Zonneveld'
if (argString === null || typeof argString === 'undefined') {
return''
  }
// .replace(/\r\n/g, "\n").replace(/\r/g, "\n");
const string = (argString + '')
let utftext = ''
let start
let end
let stringl = 0
  start = end = 0
  stringl = string.length
for (let n = 0; n < stringl; n++) {
let c1 = string.charCodeAt(n)
let enc = null
if (c1 < 128) {
      end++
    } elseif (c1 > 127 && c1 < 2048) {
      enc = String.fromCharCode(
        (c1 >> 6) | 192, (c1 & 63) | 128
      )
    } elseif ((c1 & 0xF800) !== 0xD800) {
      enc = String.fromCharCode(
        (c1 >> 12) | 224, ((c1 >> 6) & 63) | 128, (c1 & 63) | 128
      )
    } else {
// surrogate pairs
if ((c1 & 0xFC00) !== 0xD800) {
thrownewRangeError('Unmatched trail surrogate at ' + n)
      }
const c2 = string.charCodeAt(++n)
if ((c2 & 0xFC00) !== 0xDC00) {
thrownewRangeError('Unmatched lead surrogate at ' + (n - 1))
      }
      c1 = ((c1 & 0x3FF) << 10) + (c2 & 0x3FF) + 0x10000
      enc = String.fromCharCode(
        (c1 >> 18) | 240, ((c1 >> 12) & 63) | 128, ((c1 >> 6) & 63) | 128, (c1 & 63) | 128
      )
    }
if (enc !== null) {
if (end > start) {
        utftext += string.slice(start, end)
      }
      utftext += enc
      start = end = n + 1
    }
  }
if (end > start) {
    utftext += string.slice(start, stringl)
  }
return utftext
}