Convert a string to an array buffer with JavaScript

stringToBuffer.js

// Modified from https://developer.mozilla.org/en-US/docs/Web/API/TextEncoder#Polyfill

function stringToBuffer(str) {
  'use strict'
  let Len = str.length,
    resPos = -1
  // The Uint8Array's length must be at least 3x the length of the string because an invalid UTF-16
  //  takes up the equivalent space of 3 UTF-8 characters to encode it properly.
  let resArr = new Uint8Array(Len * 3)
  for (let point = 0, nextcode = 0, i = 0; i !== Len; ) {
    point = str.charCodeAt(i)
    i += 1
    if (point >= 0xd800 && point <= 0xdbff) {
      if (i === Len) {
        resArr[(resPos += 1)] = 0xef /*0b11101111*/
        resArr[(resPos += 1)] = 0xbf /*0b10111111*/
        resArr[(resPos += 1)] = 0xbd /*0b10111101*/
        break
      }
      // https://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae
      nextcode = str.charCodeAt(i)
      if (nextcode >= 0xdc00 && nextcode <= 0xdfff) {
        point = (point - 0xd800) * 0x400 + nextcode - 0xdc00 + 0x10000
        i += 1
        if (point > 0xffff) {
          resArr[(resPos += 1)] = (0x1e /*0b11110*/ << 3) | (point >>> 18)
          resArr[(resPos += 1)] =
            (0x2 /*0b10*/ << 6) | ((point >>> 12) & 0x3f) /*0b00111111*/
          resArr[(resPos += 1)] =
            (0x2 /*0b10*/ << 6) | ((point >>> 6) & 0x3f) /*0b00111111*/
          resArr[(resPos += 1)] =
            (0x2 /*0b10*/ << 6) | (point & 0x3f) /*0b00111111*/
          continue
        }
      } else {
        resArr[(resPos += 1)] = 0xef /*0b11101111*/
        resArr[(resPos += 1)] = 0xbf /*0b10111111*/
        resArr[(resPos += 1)] = 0xbd /*0b10111101*/
        continue
      }
    }
    if (point <= 0x007f) {
      resArr[(resPos += 1)] = (0x0 /*0b0*/ << 7) | point
    } else if (point <= 0x07ff) {
      resArr[(resPos += 1)] = (0x6 /*0b110*/ << 5) | (point >>> 6)
      resArr[(resPos += 1)] =
        (0x2 /*0b10*/ << 6) | (point & 0x3f) /*0b00111111*/
    } else {
      resArr[(resPos += 1)] = (0xe /*0b1110*/ << 4) | (point >>> 12)
      resArr[(resPos += 1)] =
        (0x2 /*0b10*/ << 6) | ((point >>> 6) & 0x3f) /*0b00111111*/
      resArr[(resPos += 1)] =
        (0x2 /*0b10*/ << 6) | (point & 0x3f) /*0b00111111*/
    }
  }
  return resArr.subarray(0, resPos + 1)
}