I need to move to previous input field when Up arrow is pressed
in checkbox.
I try to replace Up Arrow keypress with Shift+Tab keypress in onkeydown event.

Unfortunately, this does not work:

error: member not found

occurs at line
event.shiftKey=true;

How to move to previous input field if up arrow is pressed ?

Code to reproduce the problem:

<input type=checkbox 
onkeydown="
if(!event.ctrlKey && event.keyCode==38) {
  event.shiftKey=true;
  // next line causes error: Member not found
  event.keyCode=9;
  return; }">