Recursion in js

JavaScript
1

I dont understand the following code. Also how recursion works in js.

function a(num){
       if(num===1){return 1;}
       return num*a(num-1);
}

a(4);

for example from the code above, the way I see it was this :

  1. function checks whether num===1, if it is return 1
  2. if it is not, return num=num*(num-1);

recursion should be repetitive right? I have seen some memes/jokes about recursion is like keep repeating a part of the sentences forever and ever…

in my mind after 4*3, the result will be returned and done.

in another word I do not understand the difference between these two set of code :slight_smile:

  1. normal code :
function b(num) {
  return num * (num - 1);
}

b(4);
  1. recursion
function a(num) {
  if (num === 1) { return 1; }
  return num * a(num - 1);
}

a(4);

could someone explain this to me?
I understand that 4!=4x3x2x1, that part is easy.

2

oh wait… I think I kind of understand it now…

recursion happens when you write a function and that function calls itself

So a way I can understand this is :

return num*a(num-1); is making a function call, and this time the parameter is num-1

so what we get is this kind of operation:

  1. 4*a(3)
  2. 4*3*a(2) // the 4 times part stays the same
  3. 4*3*2*a(1) // same, the 4 times 3 part stays the same
  4. 4*3*2*1 . // because we already set a(1)===1

Is this correct???

3

Yeah here’s how i see it,

Iteration num a() returns a(num-1) Result notes
1 4 4*a(num -1) 6 24 Third fully evaluated factorial
2 3 3*a(num -1) 2 6 Second fully evaluated factorial
3 2 2*a(num -1) 1 2 First fully evaluated factorial

I think the main point is that the recursive function does not fully evaluate and return a number until num=2 , i.e. till num-1 ===1 , in all other cases of the num parameter, it will return a function.
Once it reaches that number, then notice how the result is transferred up the chain to evaluate a(num-1) . i.e. the result 2 is carried up to the 2nd iteration and the result 6 to the 1st iteration and so forth, closely resembling the mathematical definition of a factorial.

4