eventListener keypress does not work correcly for the <input>

I understand that using an keypress eventlistener for the keystrokes in an element of the input is pure madness. But I personally want to understand for myself what is I'm doing wrong in this case.

The objective of the code - is to allow only numbers to be displayed in the field input, prohibiting the appearance of letters, as well as the use of keyboard combinations (alt, shift, etc).

	function onDown(e) {
	  e = e || event;

	  if (e.ctrlKey || e.altKey || e.metaKey) return; // false for key cominations

	  var chr = getChar(e);

	  if (chr == null) return;

	  if (chr < '0' || chr > '9') { // if "chr" do not a digit - false
	    return false;
	  }
	}

	function getChar(event) { // support func to get the char from keyCode
		 if (event.which == null) { 
		   if (event.keyCode < 32) return null; 
		   return String.fromCharCode(event.keyCode)
		 }

		 if (event.which != 0 && event.charCode != 0) { 
		   if (event.which < 32) return null; 
		   return String.fromCharCode(event.which);
		 }

		 return null; 
	}

	inp.addEventListener( 'keypress', onDown );

	<input type="text"  id="inp">

Solution

You need to call e.preventDefault(); when the entered value is not a number.

function onDown(e) {
  e = e || event;
  if (e.ctrlKey || e.altKey || e.metaKey) return; // false for key cominations

  var chr = getChar(e);
  if (chr == null) return;

  if (chr < '0' || chr > '9') { // if "chr" do not a digit - false
    e.preventDefault();
  }
}

function getChar(event) { // support func to get the char from keyCode
  if (event.which == null) {
    if (event.keyCode < 32) return null;
    return String.fromCharCode(event.keyCode)
  }

  if (event.which != 0 && event.charCode != 0) {
    if (event.which < 32) return null;
    return String.fromCharCode(event.which);
  }

  return null;
}

inp.addEventListener('keypress', onDown);

<input type="text" id="inp">