Need help in understanding crockford's memoizer function

See more:

Javascript

I'm trying to understand Crockford's memoizer function. While I'm able to follow how it works, I don't quite understand the fundamental argument and the inner function shell and how he came up with fundamental(shell, n); and function(shell, n) {...}. I mean, if I were writing the code myself, what would lead me to the same conclusion?

var memoizer = function (memo, fundamental) {
  var shell = function (n) {
    var result = memo[n];
    if (typeof result !== 'number') {
      result = fundamental(shell, n);
      memo[n] = result;
    }
    return result;
  };
  return shell;
};

var fibonacci = memoizer([0, 1], function(shell, n) {
  return shell(n - 1) + shell(n - 2);
});

Intuitively, I would do something like:

function memoizer(memo, someFormula) {
  var shell = function() {
    // check memo
    // if not in memo
        // run the formula - someFormula();
        // store result in memo
    // return result;
  return shell;
  }  
}

var fibonacci = memoizer([0, 1], function(n) {
  // return fibonacci formula
};

Admittedly, it's not recursive and I've left out the n argument. While I know that shell and fundamental are not the same, I'm confused with the overlap and how they relate to each other.

What I have tried:

I've used the debugger so I understand how it works but I still don't understand the motive behind the code.