microbit.js

function onStart( ) {
   var x = 0;
   var y = 0;
   var forward = true;

   while (true) {
      if (microbit.isOn(x, y)) {
         microbit.off(x, y);
         microbit.off(4-x, y);
      }else {
         microbit.on(x, y);
         microbit.on(4-x, y);
      }
      if (forward) {
         x = x + 1;
         forward = x <= 4;
      }else {
         x = x - 1;
         forward = !( x >= 0 );
      }
      y = x;
      wait(50);
   }
}