记忆化 javascript

Question

I'm studying JavaScript from the book JavaScript: The Good Parts , in the memoization section there is an example about using memoize technique to do Fibonacci problem

We create a general function call memoizer , it takes an memo array and the fundamental function, returns a shell function that manages the memo and calls fundamental function

 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;
};

And then create fibonacci like this:

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

If I run fibonacci(10), the result will be displayed exactly.

But the thing that makes me confused is the n parameter shell function in memoizer function. I know that it is the value that we want to compute. But where is it come from? How can I call fibonacci(10), for example, can pass the value 10 to n ? And what is exactly the var fibonacci ? Is it a function or points to a function object as memoizer ?

Thanks for any help!

Answer 1

So, to understand this fully, you need to understand things below as fundamental pieces.

• Closure How do JavaScript closures work?
• Scope What is the scope of variables in JavaScript?
• First class object What is meant by 'first class object'?

So, now look at the code.
var memoizer is assigned as a function that returns an another function inside.

var memoizer = function(memo, fundamental) {
  var shell = function(n) {
    ... do some works ...
  };
  return shell;
};

Can you see it? var shell is returned at the end of the line in memoizer

And whatever the inner logic is in memoizer , the result of it is assigned to var fibonacci

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

So, it means fibonacci equals the result of memoizer (Since we excuted it), and the result of memoizer equals shell function. If you read those links given to you by me well, you can understand what is going on behind the scenes.

By the way, the code you've given isn't the best way. Because anyway closure makes an activated object alive that isn't necessary.

This code below is an another way of making fibonacci, this isn't the absolute best way , however, I recommend you to compare this code with your code, and figure the differences out.

 var result = []; result[0] = 1; result[1] = 1; function fibonacci(n){ var i; for(i = 2; i < n; i++){ if(!result[i]){ result[i] = result[i-1] + result[i-2]; } } return result[i-1]; } console.log(fibonacci(10));