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# Recursion

Functions can call themselves. This is called
recursion.
The best known example is the calculation of the factorial:

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

The factorial of a natural number is the product of all natural numbers
(excluding zero) less than or equal to that number. It is marked by a trailing exclamation mark: n! = 1 * 2 * 3 ... (n-1) * n.
You can see that you can use the factorial of n-1 to calculate the factorial of n: n! = (n-1)! * n.
The code above is the implementation of this equation. To calculate the factorial of n, you have to calculate the factorial
of n-1 and multiply the result by n. So the function calls itself repeatedly with a value reduced by 1.
Now you need a termination condition so that the recursion does not continue infinitely. This termination condition
is implemented by the `if`

condition. If n has reached 0, the function no longer calls itself, but the result,
namely 0! = 1, is returned directly.## Exercise

Write a function

Example:

`reverse`

that reverses the order of the characters in a string.
The function should be recursive.Example:

`reverse('live')`

should return `'evil'`

.