了解JavaScript memoized函数中的输入值

Question

I am currently going through 'Javascript: The good parts' by Douglas Crockford, an there is an example demonstrating the concept of memoization.

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);
});

What I don't understand is, where is the value for n coming from?

Answer 1

n is an input. It doesn't "come" from any where in the code you've posted, you have to supply it a value.

Answer 2

In the code var shell = function (n) , you're specifying that when you call the function shell , you're going to provide it with an input argument n . So if you called shell(5) , n would be equal to 5, or any other number that you passed in.

You need to look at what's being called and returned in each function call -- fibonacci is set to the returned value of the memoizer function, and memoizer returns the shell function which takes in n . So although it's never called in your example code, in the end you'd call, say, fibonacci(5) where 5 is your n . Just follow the function calls and returns.