Kotlin Extension Functions Cheat Sheet

Covers defining extension functions and properties, receiver types, extension resolution rules, and common standard library extension idioms in Kotlin.

1 PageIntermediateMar 22, 2026

Basic Extension Function

Adding a function to an existing type without subclassing it.

kotlin

// Adds a function to String without modifying its sourcefun String.isPalindrome(): Boolean {    val cleaned = this.lowercase().filter { it.isLetterOrDigit() }    return cleaned == cleaned.reversed()}println("Racecar".isPalindrome()) // true// Extension with a parameterfun Int.times(action: () -> Unit) {    repeat(this) { action() }}3.times { println("Hi") }

Extension Properties

Computed properties added to an existing type.

kotlin

// Extension properties cannot have backing fields; must be computedval String.lastChar: Char    get() = this[length - 1]println("Kotlin".lastChar) // 'n'val <T> List<T>.secondOrNull: T?    get() = if (size >= 2) this[1] else nullprintln(listOf(1, 2, 3).secondOrNull) // 2

Resolution Rules

How Kotlin decides which extension function to call.

Statically resolved- Extension functions are resolved at compile time based on the declared (static) type
Member functions win- If a class has a member with the same signature, the member always takes priority
No true polymorphism- Calling an extension on a variable typed as the base class uses the base class's extension, not the runtime type's
Nullable receiver allowed- `fun Any?.safePrint()` can be called on a null reference and check `this == null` inside
Import required- Extensions from another package must be imported (or in scope) to be visible

Scope Functions

let, run, with, apply, and also — extension functions in the stdlib.

kotlin

val result = "hello".let {    println("Length: ${it.length}")    it.uppercase() // returned as result}data class Person(var name: String, var age: Int)val person = Person("Ana", 30).apply {    age = 31 // `this` refers to the receiver; returns the receiver itself}val message = with(person) {    "$name is $age years old" // `this` is implicit inside with}person.also {    println("Created: $it") // returns the receiver, for side effects/chaining}

Common stdlib Extensions

Widely used extension functions from the Kotlin standard library.

String.trim() / isBlank() / isNotEmpty()- Common string extension helpers in kotlin.text
Iterable<T>.map/filter/fold- Higher-order extension functions on collections
Any?.let { }- Runs a block with the receiver as `it`, useful for null-checked chains
Int.coerceIn(min, max)- Clamps a numeric value within a range
File.readText()- kotlin.io extension for reading a file's contents as a String
Collection<T>.firstOrNull { }- Returns the first matching element or null instead of throwing

Pro Tip

Because extension resolution is static, never rely on an extension function to behave polymorphically across a class hierarchy — if you need runtime-type-dependent behavior, use a real member function or override, not an extension.

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