# Recursion

Now that we are familiar with functions, let's have a review on the most important concept using a function: recursion.

## We'll cover the following

## What Is Recursion?

Recursion is when a function calls itself again and again until it reaches the base condition.

### Parts of a Recursion

There are two parts of recursion: `base condition`

and `recursive function`

.

For instance, take the example of a Factorial function.

Factorial(!) of a number n is the product of all positive numbers from our chosen number n down to 1.

```
factorial(n) = 1 if n <= 1 #base case
factorial (n) = n * factorial(n-1) #recursive function
```

Let’s use an example of factorial of 5.

```
5! = 5 × 4!
= 5 × 4 × 3!
= 5 × 4 × 3 ×2!
= 5 × 4 ×3 ×2 ×1
= 120
```

Below is the call to the function factorial given 5 as a parameter to the function. factorial(5)

It is calculated recursively in the following manner:

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