From 0cc7c9467587141fd613b6263280283b5d21e09a Mon Sep 17 00:00:00 2001
From: Valerio Cipolla <83750862+ValerioCipolla@users.noreply.github.com>
Date: Fri, 6 May 2022 14:01:10 +0100
Subject: [PATCH] chore(Javascript): add factorial recursion (#756)

---
 algorithms/JavaScript/README.md                 |  4 ++++
 algorithms/JavaScript/src/index.js              |  3 +++
 .../JavaScript/src/recursion/factorial.js       | 17 +++++++++++++++++
 3 files changed, 24 insertions(+)
 create mode 100644 algorithms/JavaScript/src/recursion/factorial.js

diff --git a/algorithms/JavaScript/README.md b/algorithms/JavaScript/README.md
index 132fc3b0..f20b4faa 100644
--- a/algorithms/JavaScript/README.md
+++ b/algorithms/JavaScript/README.md
@@ -40,3 +40,7 @@
 ## Maths
 
 - [Fibonacci Series](src/maths/fibonacci-series.js)
+
+## Recursion
+
+- [Factorial](src/recursion/factorial.js)
diff --git a/algorithms/JavaScript/src/index.js b/algorithms/JavaScript/src/index.js
index 806026de..b15a3328 100644
--- a/algorithms/JavaScript/src/index.js
+++ b/algorithms/JavaScript/src/index.js
@@ -7,6 +7,9 @@ require('./linked-lists/singly');
 // Maths
 require('./maths/fibonacci-series');
 
+// Recursion
+require('./recursion/factorial');
+
 // Searching
 require('./searching/binary-search-recursive');
 require('./searching/binary-search');
diff --git a/algorithms/JavaScript/src/recursion/factorial.js b/algorithms/JavaScript/src/recursion/factorial.js
new file mode 100644
index 00000000..e3065f29
--- /dev/null
+++ b/algorithms/JavaScript/src/recursion/factorial.js
@@ -0,0 +1,17 @@
+// algorithm to calculate factorial of positive integer n
+// Time complexity: O(n)
+
+function factorial(n) {
+  // Error handling
+  // if n is negative or not an integer return string with error message
+  if (n < 0) return 'Only positive numbers allowed';
+  if (!Number.isInteger(n)) return 'Only integers allowed';
+  // if n is a positive integer
+  // base case: we reach 0, return 1
+  if (n === 0) return 1;
+  // until we reach 0 we keep multiplying n by the factorial of n - 1
+  return n * factorial(n - 1);
+}
+
+console.log(factorial(5));
+// output: 120