How does JavaScript solve UTF8 encoding?

Javascript| Code <script>
Alert (Chinesefromutf8url ("%e4%b8%ad%e6%96%87"))
function Utf8codetochinesechar (STRUTF8)
{
var icode, ICode1, ICode2;
Icode = parseint ("0x" + strutf8.substr (1, 2));
ICode1 = parseint ("0x" + strutf8.substr (4, 2));
ICode2 = parseint ("0x" + strutf8.substr (7, 2));

Return String.fromCharCode ((Icode & 0x0f) << 12) |
((ICode1 & 0x3F) << 6) |
(ICode2 & 0x3F));
}

Converts the Chinese portion of the UTF8 encoded URL (< 0xFFFF).
function Chinesefromutf8url (STRUTF8)
{
var BSTR = "";
var noffset = 0; Processing point on StrUtf8

if (StrUtf8 = "")
Return "";

StrUtf8 = Strutf8.tolowercase ();
Noffset = Strutf8.indexof ("%e");
if (Noffset = = 1)
return StrUtf8;

while (Noffset!=-1)
{
BSTR + + strutf8.substr (0, Noffset);
StrUtf8 = Strutf8.substr (Noffset, Strutf8.length-noffset);
if (StrUtf8 = = "" | | Strutf8.length < 9)//bad string
Return BSTR;

BSTR + = Utf8codetochinesechar (strutf8.substr (0, 9));
StrUtf8 = Strutf8.substr (9, strutf8.length-9);
Noffset = Strutf8.indexof ("%e");
}

Return BSTR + StrUtf8;
}

function UnicodeFromUtf8 (STRUTF8)
{
var BSTR = "";
var ntotalchars = strutf8.length; Total chars to be processed.
var noffset = 0; Processing point on StrUtf8
var nremainingbytes = Ntotalchars; How many bytes left to be converted
var noutputposition = 0;
var icode, ICode1, ICode2; The value of the Unicode.

while (Noffset < ntotalchars)
{
Icode = Strutf8.charcodeat (Noffset);
if ((Icode & 0x80) = = 0)//1 byte.
{
if (Nremainingbytes < 1)//Not enough data
Break

BSTR + + String.fromCharCode (Icode & 0x7F);
Noffset + +;
Nremainingbytes-= 1;
}
else if ((Icode & 0xe0) = = 0xc0)//2 bytes
{
ICode1 = strutf8.charcodeat (Noffset + 1);
if (Nremainingbytes < 2 | |//Not enough data
(ICode1 & 0xc0)!= 0x80)//Invalid pattern
{
Break
}

BSTR + + String.fromCharCode ((Icode & 0x3F) << 6) | (ICode1 & 0x3F));
Noffset + 2;
Nremainingbytes-= 2;
}
else if ((Icode & 0xF0) = = 0xe0)//3 bytes
{
ICode1 = strutf8.charcodeat (Noffset + 1);
ICode2 = strutf8.charcodeat (Noffset + 2);
if (Nremainingbytes < 3 | |//Not enough data
(ICode1 & 0xc0)!= 0x80 | | Invalid pattern
(ICode2 & 0xc0)!= 0x80)
{
Break
}

BSTR + + String.fromCharCode ((Icode & 0x0f) << 12) |
((ICode1 & 0x3F) << 6) |
(ICode2 & 0x3F));
Noffset + 3;
Nremainingbytes-= 3;
}
else//4 or more bytes-unsupported
Break
}

if (nremainingbytes!= 0)
{
Bad UTF8 string.
Return "";
}

Return BSTR;
}
</script>