KeyPress n KeyUp events

function fn() {
    var element = document.getElementById("NumberInput");
    element.value = "";
    element.onkeydown = function (e) {

        if (element.value.length == 0) {
            element.value = "M";
        }

    };

    element.onkeyup = function (e) {

        if (e.keyCode == 8) {
            var Number = element.value;
            var caretPos = element.selectionStart;
            if (caretPos == element.value.length) {
                Number = Number.replace('M', '');
                Number = Number.substring(0, Number.length - 1);
                element.value = Number + 'M';
            }
        }
        else {
            var Number = element.value;
            Number = Number.replace('M', '');
            element.value = Number + 'M';
        }
    };
}