easy_error

/* can you spot what is happening here */
/* the alert("X") should never execute */
<script>
onload = function() {

  var fn = function() {
    alert("X");
  }

  (function() { })();
}

/*

1) Semicolon after first function expression is missing, so overall expression is equivalent to:

var fn = function(){ alert('X'); }(function(){})();

2) Which is essentially double invocation of a function and an assignment of its result to `fn`:

var fn = function(){ alert('X'); }()();
                                  ^^^^
3) an error should be thrown since first invocation returns `undefined`, making and expression equivalent to:

var fn = (undefined)(); // Error

*/
</script>