Edit in JSFiddle

var str1 = "Hello World";
var str2 = "아름다운 한글";

var res1 = str1.charAt(0); // H
var res3 = str2.charAt(2); // 다

var res2 = str1.charCodeAt(0); // 72
var res4 = str2.charCodeAt(2) // 0xB2E4

var res5 = String.fromCharCode(65,66,67,0xAC00,0xAC01); // ABC가각

//출력
document.write(res1 + " : " + res2 + "<br>"); // H : 72
document.write(res3 + " : 0x" + res4.toString(16).toUpperCase() + "<br>"); // 다 : 0xB2E4
document.write(res5); // ABC가각