Something like this (not tested):
function HexToInt(Hex: string): integer;
var
n: integer;
begin
result:=0;
// if more than 8 hex digits, the first ones must be 0
if length(Hex)>8 then
begin
for n:=1 to length(Hex)-8 do
begin
if Hex[n]<>'0' then
raise EConvertError.CreateFmt('Hexadecimal value 0x%s out of range',[Hex]);
end;
end;
for n:=1 to length(Hex) do
begin
result:=result shl 4;
case UpCase(Hex[n]) of
'0' : begin end;
'1'..'9' : Inc(result,ord(Hex[n])-48); // 1 - 9
'A'..'F' : Inc(result,ord(Hex[n])-55); // 10 - 15
else
raise EConvertError.CreateFmt('Invalid hexadecimal digit: %s',[Hex[n]]);
end;
end;
end;
// IntToHex is built in (include SysUtils)
function toHex(s: string): string;
var
n: integer;
begin
for n:=1 to length(s) do
begin
result:=result+'%'+IntToHex(ord(s[n]),2);
end;
end;
function toAscii(s: string): string;
var
n: integer;
begin
result := '';
n := 1;
while n <= length(s) do
begin
if s[n]='%' then
begin
result:=result+chr(HexToInt(copy(s,n+1,2)));
Inc(n,2);
end
else
result:=result+s[n];
Inc(n);
end;
end;
Obviously, you need to get the values from somewhere (say, a TEdit). The HexToInt function is general purpose - it works for all 32 bit hex values. The toAscii function too - it will allow parts to be unencoded (say, "hel%6Co" = "hello").
toHex converts ALL characters to hex - just like the javascript function - it doesn't care if they could more efficiently be simply plain ascii characters. No idea why the function doesn't use escape(inValue) (or URIescape(inValue)), but I just stuck with what the original function does , since I don't know what you're going to use it for 
. For URL encoding, it should be tested if the characters are legal, before encoding them. Something like:
function toHex(s: string): string;
var
n: integer;
begin
for n:=1 to length(s) do
begin
if not (s[n] in ['a'..'z','A'..'Z','0'..'9','-','_','.','!','*',#39,'(',')']) then
result:=result+'%'+IntToHex(ord(s[n]),2)
else
result:=result+s[n];
end;
end;
You may want to remove all the special characters ('-','_' etc.) from the list above. They're allowed (but not certain to be "safe") in URLs. Also remember that this function should not be used to encode the entire URL with host and path - just for encoding values to be sent via HTTP GET But again, don't know if that's what you want it for.