Kotlin extension functions (#31943)

guide/english/kotlin/extension-functions/index.md

@@ -0,0 +1,46 @@
+---
+title: Extension Functions
+---
+
+# Extension Functions
+
+Kotlin supports extension functions & extension variables, which allow you to add new functionality to a class without modifying the class itself.
+
+### Basic Usage
+
+#### Extension Function
+
+```kotlin
+fun String.containsSpaces(): Boolean {
+  return contains(" ")
+}
+
+val myString = "Hello World"
+
+print(myString.containsSpaces())
+```
+
+Output:
+```shell
+true
+```
+
+#### Extension Variable
+
+```kotlin
+val <T> List<T>.lastIndex: Int
+    get() = size - 1
+
+val myList = listOf(1,2,3,4,5)
+
+print(myList.lastIndex)
+```
+
+Output:
+```shell
+4
+```
+
+#### Resources
+
+* [Kotlin Extensions](https://kotlinlang.org/docs/reference/extensions.html)