This is to ensure that `document.body` is not accessed prematurely. If the script were to be included in the `<head>`, accessing `document.body` would throw an error. `supported` is also memoised, to prevent the check happening more than once.

However, since it is only being called once, perhaps this is prematurely optimised. So how about:

    var inputSupportedIE = function() {
      return "oninput" in document.body && ieVersion !== 9);
    };

(Note: not immediately invoked)