/*jshint laxcomma:true */

​

var input = document.querySelector('#calc')

  , button = document.querySelector('#calculate');

​

input.addEventListener('keypress', function(e) {

  var key = String.fromCharCode(e.which);

  if (!/[0-9+\-/*]+/.test(key)) e.preventDefault();

});

button.addEventListener('click', function() {

  alert(input.value +' = '+ eval(input.value));

});