JavaScript functions to convert UTF-8 string to hexadecimal string, and back. Correctly handle multi-byte. Demo https://jsfiddle.net/lyquix/k2tjbrvq/

utf8_hex.js

function utf8_hex(str) {
  return Array.from(str).map(c => 
	c.charCodeAt(0) < 128 ? c.charCodeAt(0).toString(16).padStart(2, '0') :
	encodeURIComponent(c).replace(/\%/g,'').toLowerCase()
  ).join('');
}

function hex_utf8(hex) {
  return decodeURIComponent('%' + hex.match(/.{1,2}/g).join('%'));
}