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JavaScript

Number.prototype.getHangul = function() {
    var numberic = ["","일","이","삼","사","오","육","칠","팔","구"];
    var numunit = ["","","","","천","만","십만","백만","천만","억","십억","백억","천억"];
    var str = "", tmp = "";
    
    var splited = [];
    for(var i = 0; i < String(this).length; i ++) {
        splited.push(String(this).substring(i, i+1));
    }

    for(var i = 0, x = String(this).length; x > 0; -- x, ++ i) {
        tmp = numberic[splited[i]];
        if(tmp) {
            if(x > 4 && numberic[splited[i + 1]]) {
                tmp += numunit[x].substring(0, 1);
            } else {
                tmp += numunit[x];
            }
        } else {
            tmp += "";
        }
        str += tmp;
    }
    
    return str;
};

var num = 6642;
console.log(num.getHangul());