Once an IIFE expression is stored in a variable, why can’t the function be invoked again?

I have:

var Init = (function() {
   my js goes here
})();

And my JS executes correctly when the page is loaded. I also have:

$('form :checkbox').change(function() {
   Init();
});

But Firebug says Init is not a function.

Solution

It isn't a function.

(function() {
   ...
})()

evaluates the anonymous function right then. And the result of the evaluation apparently does not return a function-object in this case :-)

Consider:

f = (function() {
   return "not a function :("
})()
alert(f())

and

f = (function() {
   return function () { return "Yay!" }
})()
alert(f())

Happy coding :)

Here is a function which will "execute something once" and then "return that something to execute later". (See "You can either [assign] a function or call it; you can't do both..." from Slaks answer.) However, I wouldn't do it like this.

Init = (function () {
  function Init () {
    alert("whee!")
  }
  Init()
  return Init
})()
Init()

Here is another solution (much shorter/cleaner) from CD Sanchez (see comment) which takes advantage of the fact that an assignment evaluates to the assigned value:

var Init; (Init = function Init () {
  alert ("wee");
})()