cereallarceny - 1 year ago 90
Javascript Question

# Javascript closures and recursion

I've been reading more specifically into the closure concept of programming, particularly pertaining to Javascript. I'm not quite there yet in understanding how it's any different from the Javascript code I've been writing for years though. I understand the concept of recursion as well, but I'm wondering, how are closures and recursion similar? Do I understand correctly that recursion, in and of itself, is a type of closure?

Closure:

``````function init() {
var name = "Stack Overflow";
function displayName() {
}
displayName();
}
init();
``````

Recursion:

``````function factorial(num) {
if(num < 0)
return -1;

else if(num == 0)
return 1;

else
return (num * factorial(num - 1));
}

``````

I think I'm beginning to understand that a closure is nothing more than having a function within a function, with the inside function having access to the outside function via scoping. Could there be a recursive closure? I mean, while my example of recursion isn't exactly an example of closure as well, could it at least happen? I'm trying to understand how recursion and closures are similar, different, or if they're even comparable by all. Are there any examples to maybe describe this?

A closure is simply a function that "closes over" an environment.

Recursion is the effect of a procedure repeating itself

I've been reading a new book and it talks about some relevant topics. I thought I'd just share some notes here in case you were interested.

Here's a recursive function for calculating factorials. It does the same thing as yours but in a very different way. Let's see!

``````function factorial(x) {
if (x < 0) throw Error("Cannot calculate factorial of a negative number");
function iter(i, fact) {
return i === 0 ? fact : iter(i-1, i*fact);
}
return iter(x, 1);
}

factorial(6); // 720
``````

Compare it to the one you defined above ^.^. Notice that even though it's recursive, it doesn't call itself (`factorial`) again. This uses a linear iterative process that consumes linear space and time. The evaluation of the function looks something like this

``````factorial(6);
iter(6, 1);
iter(5, 6*1);
iter(4, 5*6);
iter(3, 4*30);
iter(2, 3*120);
iter(1, 2*360);
iter(0, 1*720);
// => 720
``````

Compare that to the process of your function. Here's what yours would look like

``````factorial(6);
(6 * factorial(5));
(6 * (5 * factorial(4)));
(6 * (5 * (4 * factorial(3))));
(6 * (5 * (4 * (3 * factorial(2)))));
(6 * (5 * (4 * (3 * (2 * factorial(1))))));
(6 * (5 * (4 * (3 * (2 * 1)))));
// => 720
``````

Notice that as `n` increases, `n!` adds another stack call. This is a linear recursive process. For large values of `n`, this method uses significantly more space to calculate the same result.

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