diff --git a/Maths/Factorial.js b/Maths/Factorial.js
index 7dc6fd6d6f..22285c254a 100644
--- a/Maths/Factorial.js
+++ b/Maths/Factorial.js
@@ -11,6 +11,12 @@
 https://en.wikipedia.org/wiki/factorial
 */
 
+/**
+ * Returns the factorial of a number n.
+ * Factorial is the product of all positive integers up to n.
+ * Example: factorial(5) = 5 <span class="naked_sign"><span class="naked_sign">×</span><span class="naked_aural">ばつ</span></span><span class="naked_aural">かける</span> 4 <span class="naked_sign"><span class="naked_sign">×</span><span class="naked_aural">ばつ</span></span><span class="naked_aural">かける</span> 3 <span class="naked_sign"><span class="naked_sign">×</span><span class="naked_aural">ばつ</span></span><span class="naked_aural">かける</span> 2 <span class="naked_sign"><span class="naked_sign">×</span><span class="naked_aural">ばつ</span></span><span class="naked_aural">かける</span> 1 <span class="naked_sign">=</span><span class="naked_aural">わ</span> 120
+ */
+
 'use strict'
 
 const calcRange = (num) => {
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