Recursive functions in JavaScript

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

• Daniel van Dommele: shortest would be "base ^ exponent" :)
• Reinstate Monica -- notmaynard: (To comment above: actually, ^ is exclusive or, not power.)

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.

• tkone: @Sam Huh? Explain how to access values that are members of an array? That's all that it is doing. Let's say exponent = 5, so that call translates into stack[4], accessing the fifth element of the array (array indexes start at 0). Mozilla documentation for array. They're both identical in nature, just one of them sets a value (the one with the =) and one of them gets a value.

Answer 2

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

• Sam: Ohh, so it runs completely and then returns. I was wondering how it would be correct of it just kept returning! Thanks so much
• Sandeep Manne: when you call a function 1 and function 1 calls function 2 and function 2 called function 3, response will come first from function 3 then function 2 then function 1, its a stack en.wikipedia.org/wiki/Stack_(data_structure)

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.

• Daniel van Dommele: I think that would be the other way around. stack[0] being 2 and stack[2] being 8.
• Sam: How does it return all of them at once? That's the part confusing me to make the stack array complete.
• Daniel Ribeiro: That's exactly my point. You shouldn't return that statement. It's wrong.

Answer 4

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?

• John Dvorak: Is this an answer, or a question? If it's a question, please post it as such. If it's an answer, I can't see how it answers the question. You could post the question on Code Review or Code Golf (as a challenge), perhaps.