Codificação e decodificação UTF8 com Javascript

  function utf8Decode(utf8String) {
      if (typeof utf8String != 'string') 
      throw new TypeError('parameter ‘utf8String’ is not a string');

      const unicodeString = utf8String
      .replace(/[\u00e0-\u00ef][\u0080-\u00bf][\u0080-\u00bf]/g,(c) => {
        return String.fromCharCode(((c.charCodeAt(0)&0x0f)<<12) 
        | ((c.charCodeAt(1)&0x3f)<<6) 
        | ( c.charCodeAt(2)&0x3f)); })
      .replace(/[\u00c0-\u00df][\u0080-\u00bf]/g, (c) => {
        return String.fromCharCode((c.charCodeAt(0)&0x1f)<<6 
        | c.charCodeAt(1)&0x3f); 
      });
      return unicodeString;
  }

  function utf8Encode(unicodeString) {
      if (typeof unicodeString != 'string') 
      throw new TypeError('parameter ‘unicodeString’ is not a string');

      const utf8String = unicodeString
      .replace(/[\u0080-\u07ff]/g,(c) => {
        let cc = c.charCodeAt(0);
        return String.fromCharCode(0xc0 | cc>>6, 0x80 | cc&0x3f); })
      .replace(/[\u0800-\uffff]/g,(c) => {
        let cc = c.charCodeAt(0);
        return String.fromCharCode(0xe0 | cc>>12, 0x80 | cc>>6&0x3F, 0x80 | cc&0x3f); 
      });
      return utf8String;
  }

const encodeUTF8 = (s) => unescape(encodeURIComponent(s));
const decodeUTF8  = (s) => decodeURIComponent(escape(s));
console.log(encodeUTF8('Açaí')) // "Açaí"
console.log(decodeUTF8("Açaí")) // "Açaí"