JavaScript中的递归函数

Question

Please help me understand this recursive function...

var stack = Array;
function power(base, exponent){
    if ( exponent === 0 ) {
        return 1;
    } else {
        stack[exponent-1] = base * power(base, exponent - 1);
        return stack[exponent-1];
    }
}

I dont understand what

stack[exponent-1]

is doing

Answer 1

Which one? It's called twice. But each time it's either getting the value that exists in the array at the index equal to the current value of exponent-1 or setting that value.

It's just an array index and access.

Answer 2

have a look on this version!

function pow(x,n)
{
     return n==0?1:n==1?x:n==2?x*x:pow(pow(x,(n-n%2)/2),2)*(n%2==0?1:x);
}

can get it shorter?

Answer 3

The algorithm is stacking the result of each power from the initial exponent to 0.

If you run power(2, 3) , the stack will, at some point, be:

stack[2] = 8
stack[1] = 4
stack[0] = 2

This really has nothing to do with the mathematic concept of power.

Answer 4

I did console log of stack[exponent-1] using

var stack = Array;
function power(base, exponent){
    if ( exponent === 0 ) {
        return 1;
    } else {
        stack[exponent-1] = base * power(base, exponent - 1);
        console.log(stack[exponent-1]);return stack[exponent-1];
    }
}

O/P:

power(2,5)
2
4
8
16
32

So function class recursively until exponent become 0 (nth call), then it will start returning results

first it will return 1     (because exponent is 0)
        then returns 2 * 1 (return of n call)
              then   2 * 2 (return of n-1 call) 
              then   2 * 4 (return of n-2 call) and so on