The javascript definition of isupper is defined here:

function isupper(a) {
  return /^[A-Z_$]*$/.test(a);
}

This is slightly different from the Python implementation, with respect to digits:

'__IGNORE_0'.isupper()
# True

/^[A-Z_$]*$/.test('__IGNORE_0');
// false

The regex should be updated to /^[A-Z0-9_$]*$/