Edit in JSFiddle

var text = "古"
var con = document.getElementById('rs');
var is32Bit = function(c){
  return c.codePointAt(0) > 0xFFFF;
}
var codePointLength = function(s){	// so slow if more then long length
	var rs = s.match(/[\s\S]/gu)
  return rs ? rs.length : 0;
}
var hasRegExpU = function(){
	try{
  	var pattern = new RegExp(".", "U");
    return true
  }catch(e){
  	return false	// is not U flag
  }
}


con.textContent = text;
con.textContent += "\n"
con.textContent += String.fromCodePoint(134071)
con.textContent += "\n" + '#'.repeat(64) + "\n"
con.textContent += text.length
con.textContent += "\n"
con.textContent += "/^.$/.test(text): " + /^.$/.test(text)
con.textContent += "\n"
con.textContent += "/^.$/.test(String.fromCodePoint(134071)): " + /^.$/.test(String.fromCodePoint(134071))
con.textContent += "\n"
con.textContent += "/^.$/.test(String.fromCodePoint(134071)): " + /^.$/.test(String.fromCodePoint(134071))
con.textContent += "\n"
con.textContent += "/^.$/u.test(String.fromCodePoint(134071)): " + /^.$/u.test(String.fromCodePoint(134071))	// u is unicode char regexp
con.textContent += "\n" + '#'.repeat(64) + "\n"
con.textContent += "text.charAt(0): " + text.charAt(0)||'undefined'
con.textContent += "\n"
con.textContent += "text.charAt(1): " + text.charAt(1)||'undefined'
con.textContent += "\n"
con.textContent += "text.charAt(2): " + text.charAt(2)||'undefined'
con.textContent += "\n" + '#'.repeat(64) + "\n"
con.textContent += "text.charCodeAt(0): " + text.charCodeAt(0)||'undefined'
con.textContent += "\n"
con.textContent += "text.charCodeAt(1): " + text.charCodeAt(1)||'undefined'
con.textContent += "\n"
con.textContent += "text.charCodeAt(2): " + text.charCodeAt(2)||'undefined'
con.textContent += "\n" + '#'.repeat(64) + "\n"
con.textContent += "text.codePointAt(0): " + text.codePointAt(0)||'undefined'
con.textContent += "\n"
con.textContent += "text.codePointAt(1): " + text.codePointAt(1)||'undefined'
con.textContent += "\n"
con.textContent += "text.codePointAt(2): " + text.codePointAt(2)||'undefined'
con.textContent += "\n" + '#'.repeat(64) + "\n"
con.textContent += "is32Bit('古'): " + is32Bit("古");
con.textContent += "\n"
con.textContent += "is32Bit('a'): " + is32Bit("a");
con.textContent += "\n"
con.textContent += "is32Bit(String.fromCodePoint(134071)): " + is32Bit(String.fromCodePoint(134071));
con.textContent += "\n" + '#'.repeat(64) + "\n"
con.textContent += "String.fromCodePoint(21476): " + String.fromCodePoint(21476)
con.textContent += "\n"
con.textContent += "String.fromCodePoint(134071): " + String.fromCodePoint(134071)
con.textContent += "\n" + '#'.repeat(64) + "\n"
con.textContent += 'codePointLength("abc"): ' + codePointLength("abc") // 3
con.textContent += "\n"
con.textContent += 'codePointLength("古bc"): ' + codePointLength("古bc") // 3
con.textContent += "\n"

<pre id="rs">

</pre>