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1. Add McJson.pas into project
2. Add mcjson into uses clause
3. Add variable and assign it as TMcJsonItem (ie: var js : tmcjsonitem; )
4. Create the variable (ie: var := TMcJsonItem.create(); )
5. free the variable when not in use

var
  Form1: TForm1;
  js : TMcJsonItem;
  
procedure TForm1.FormShow(Sender: TObject);
begin
   js := TMcJsonItem.create();
end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
   js.Free;
end;

Add into json variable

   Js.Add('key1').AsInteger := 1;
   Js.Add('key2').AsBoolean := True;
   Js.Add('key3').AsNumber  := 1.234;
   Js.Add('key4').AsString  := 'value 1';
   // add an array
   Json.Add('array', jitArray);
   for i := 1 to 3 do Js['array'].Add.AsInteger := i;

Check if a key exist

   if js.HasKey('key1') then MessageDlg('Key found');

To update value of key

   if js.HasKey(key) then begin
      js.Delete(key);
      js.Add(key).AsString := value;
   end;

To get value of key

   ShowMessage(js.Values[key].AsString);