Turkish toUpperCase function for Javascript

Javascript toUpperCase function for Turkish;

String.prototype.toTrkUppercase = function() {
    var str = [];
    for(var i = 0; i < this.length; i++) {
        var ch = this.charCodeAt(i);
        var c = this.charAt(i);
        if(ch == 105) str.push('İ');
        else if(ch == 305) str.push('I');
        else if(ch == 287) str.push('Ğ');
        else if(ch == 252) str.push('Ü');
        else if(ch == 351) str.push('Ş');
        else if(ch == 246) str.push('Ö');
        else if(ch == 231) str.push('Ç');
        else if(ch >= 97 && ch <= 122)
            str.push(c.toUpperCase());
        else
            str.push(c);
    }
    return str.join('');
}

////Sample;
//alert("ğüşiöçıĞÜŞİÖÇI".toTrkUpperCase())
////Output: ĞÜŞİÖÇIĞÜŞİÖÇI