# Why is My Recursive Function Returning Undefined in JavaScript?

Nick Scialli • November 13, 2020 • 🚀 1 minute read

One common frustration when getting the hang of recursion is not understanding why your recursive function is returning undefined.

Often, this is because you’re not `return`

ing the next execution of your recursive function. I’ll show you what I mean.

Let’s say we’re writing a factorial function. Factorial is a mathematical operator where a number is multiplied by each lower number. For example, 4 factoral (written as 4!) is equivalent to 4 * 3 * 2 * 1 = 24.

We might write a recursive factorial function like this:

```
function factorial(num, result = 1) {
if (num === 1) {
return result;
}
factorial(num - 1, num * result);
}
console.log(factorial(4));
// undefined
```

But this should be `24`

! Why is it `undefined`

? Well the answer is easier than we think: we need to `return`

the next execution of the `factorial`

function each time or else we’ll never get to the ultimate result!

```
function factorial(num, result = 1) {
if (num === 1) {
return result;
}
return factorial(num - 1, num * result);
}
console.log(factorial(4));
// 24
```

And that’s really it! The “secret” is to just make sure that, whatever path you go down in your function, make sure you’re `return`

ing something.

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**Nick Scialli** is a software engineer at the U.S. Digital Service.