Restricting keys allowed to an input

numbers_only.js

document.getElementById('numbersonly').addEventListener(
  'keypress', 
  function() {
    var allowed_keys = {
      48: true,  // 0
      49: true,  // 1
      50: true,  // 2
      51: true,  // 3
      52: true,  // 4
      53: true,  // 5
      54: true,  // 6
      55: true,  // 7
      56: true,  // 8
      57: true,  // 9
       8: true,  // backspace
       9: true,  // tab
      13: true,  // enter
     127: true,  // delete
      32: false  // space
    };
    
    return function(event) {
      var key = document.all ? event.keyCode : event.which;
      if (!allowed_keys[key]) {
        event.preventDefault();
      }
    };
    
  }(), 
  false
);