coding basics if / else and return (for example in javascript)

Question

I have been some time coding, not to much as you will see (1-2 years), but I am facing this question and I am feeling ashamed. This is not the first time I am confused about it. I have looked in some books of other languages as Java, but I can't solve it by myself.

Why in this script the return is undefined?

const factorial = n => {
  let result = 1;
  const rec = (n) => {
    if(n>0) {
    result = result * n;
    rec(n-1);
    } else {
    console.log(result)
    return result;
    }
  }
  return rec(n);
};

console.log(factorial(5))
120
undefined

PS: I edited to clear what I would like to understand. Why function rec is not returning result?

If you remove the console.log from within factorial, what does it do? What effect does the line factorial(n-1); have? — Caleth
– Caleth, Commented Dec 20, 2022 at 10:24
main thing I would like to understand: why return is not working in "else". Whatever I write and return, it returns undifined. — titoih
– titoih, Commented Dec 20, 2022 at 10:34
The answer is that factorial(n-1); does nothing without the console.log. In the call factorial(5), there is no return, control falls off the bottom of the function — Caleth
– Caleth, Commented Dec 20, 2022 at 10:36
I made some edition. I am returning in the else, why function rec is not returning this value? — titoih
– titoih, Commented Dec 20, 2022 at 10:40
Because the call to rec(5) doesn't visit the else. Only the call to rec(0) does — Caleth
– Caleth, Commented Dec 20, 2022 at 10:41

Caleth · Accepted Answer · 2022-12-20 10:52:02Z

1

Your function has multiple calls. Either return from all of them, or none of them.

const factorial = n => {
  if(n>0) {
    return factorial(n-1) * n;
  } else {
    return 1;
  }
};


const factorial = n => {
  let result = 1;
  const rec = (n) => {
    if(n>0) {
      result = result * n;
      rec(n-1);
    }
  }
  rec(n);
  return result;
};

The alternative is to loop, not recurse

const factorial = n => {
  let result = 1;
  for (; n>0; n--) {
    result = result * n;
  }
  return result;
};

answered Dec 20, 2022 at 10:52

Caleth

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2 Comments

titoih Over a year ago

I am really glad and appreciate your time. I am still a little bit confused why else return is undefined and not result value if it is in the end of the function, but I understand better how to make recursion. Thanks for your help!

Caleth Over a year ago

@titoih the function happens multiple times, each is independent of the others. returning from the innermost function doesn't mean the outermost function returns.

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