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

Example: reverse('live') should return 'evil'.