 # Factorial challenge

Hi, so I was practising the 3 challenges for loops and arrays at the end of Javascript part I.
When I faced the factorial challenge, I did a search for a solution to calculate the factorial.

I’ve found a function expression that solves it, but I couldn’t figure out what exactly was going on.
The expression is:

const factorial = function fac(n) {return n < 2 ? 1 : n * fac(n - 1);

I understand that first it checks if the given argument is less than 2, so it would return 1, and if not it calculates the factorial.

I didn’t get the placeholder “fac(n)”, I know could be any other name like “factorial(x)”, but couldn’t understand how javascript is calculating the result exactly, can someone clarify?

Hello, @efc.

If you’ve not yet studied recursion, this solution will be more difficult to understand. If you stay the course, you’ll get to recursion, and the code you posted will make sense.

In a nutshell, recursion is calling a function from within itself. Doing so builds a ‘call stack’ where each function call is waiting for the result of the subsequent function call to be returned.

In the code you posted, a function `fac` is defined which takes a parameter, `n`. If `n < 2` then `1` is returned. Otherwise the result of multiplying `n` by the return value of calling the same function, `fac` on 1 less than `n` is returned.

Here’s essentially the same function using `if...else` rather than the ternary expression with a couple of `console.log()` statements added to help illustrate.

function fac(n){ if(n < 2){ console.log(`\${n} is less than 2, so returning 1`) return 1; } console.log(`returning result of \${n} * fac(n - 1)`) return n * fac(n - 1); }; console.log(fac(4)); //same function written with ES6's concise body arrow function syntax const fac2 = n => n < 2 ? 1 : n * fac2(n - 1); console.log(fac2(4));

I wouldn’t get too caught up in understanding recursion just yet. It will come in time.

Thank you so much for taking your time to answer me, it was a really clear and helpful answer. I was worried that I was missing something, if I didn’t learn it yet that’s fine, I will solve it using a loop for now.

Best!
Erick

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Recursion is really cool once you get there. Happy coding!

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There is one assumption that goes with that expression… `n` is not negative. 0! and 1! are both 1, but there is no solution for a negative n.

Something else of note in the above expression, the named function, something we would rarely see unless the function is recursive.

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