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JavaScript

Posted by Stephen O'Connor
Stephen O'Connor
22,291 Points

JavaScript Function Explanation

I am (once again) trying to get my head around JavaScript and I am currently working through the MDN guide and in the functions section there is some code I do not understand. Can someone explain to me how it works?

function factorial(n){
  if ((n == 0) || (n == 1))
    return 1;
  else
    return (n * factorial(n - 1));
}

var a, b, c, d, e;
a = factorial(1); // a gets the value 1
b = factorial(2); // b gets the value 2
c = factorial(3); // c gets the value 6
d = factorial(4); // d gets the value 24
e = factorial(5); // e gets the value 120

I don't understand how the function evaluates to the numbers in the comments ... What I see is (with d for example): n=4 so the function returns 4 * (4-1), which is 4 * 3, which equals 12, not 24??

I don't get it.

Thanks!

Ted Sumner
Ted Sumner
Courses Plus Student 17,967 Points

Duplicate question deleted.

1 Answer

Ted Sumner
Ted Sumner
Courses Plus Student 17,967 Points

It is calling itself, so when you enter 3, 3 is not 0 or 1, so it does the else. The else takes 3 and multiplies by the function return of 3-1 or 2. When it executes on 2, it again calls itself for 1, which returns 1 and does not call the function again.

So you have

n * (n-1) * (n-1-1) = 6 where n is 3

Justin Iezzi
Justin Iezzi
18,199 Points

I want to add that a function called within itself is an example of recursion. So this would be a recursive function.

Stephen O'Connor
Stephen O'Connor
22,291 Points

Hi Ted,

Thanks for your response. I guess that clears it up a little ... but would the function not always just return 1 then? Because n at some point will always be 1 and the conditional states that if it is 1 then the function returns 1?

Apologies if these questions are dumb, I have been trying to 'get' JavaScritp for a while now, but I just can't seem to grasp it at all.

Ted Sumner
Ted Sumner
Courses Plus Student 17,967 Points

It would not just return 1. It would return this using 3 as an example:

Return (3 * (return 2 * (return 1)))

Looking at my answer from 2 am, it is not that clear.

3 is not 0 or 1, so it does the else and multiplies 3 and the result of the function for 3-1 = 2. When it puts 2 in the code, it is not 0 or 1, so it returns 2 * the result of the function for 2-1=1. To work from inside out, you get 1*2*3=6.

This is not a JavaScript issue. This is a recursive function issue. I think most languages have recursive functions, but I am not sure. Let me know if you still don't understand and I will come up with a different explanation.

Stephen O'Connor
Stephen O'Connor
22,291 Points

Thanks Ted, that clears it up better, trying to get my head around JavaScript, or just programming in general has been a big challenge for me, and I guess there will be other times when I am scratching my head wondering what the hell is going on, but I will stick at it.

Thanks again.

Ted Sumner
Ted Sumner
Courses Plus Student 17,967 Points

What I do when I don't understand code like this is to think through the code as if I was an example input. Using n=3 is a good example. I go through each line of code replacing n with 3 from the beginning. But instead of reading from top to bottom, I read from the top until there is a jump. I follow the jump because the 3 doesn't care about code not being applied to it. That way I learn the logic of the code. In more complicated examples I will take notes with line numbers if I have them.