plus

Concatenates this string with the string representation of the given other object. If either the receiver or the other object are null, they are represented as the string "null".

expect operator fun String?.plus(other: Any?): String(source)

fun main() {
    val greeting: String? = "Hello"
    val name: Any? = "World"
    println(greeting + name) // Prints: HelloWorld

    val nullString: String? = null
    val number: Any? = 42
    println(nullString + number) // Prints: null42

    val kotlinStr: String? = "Kotlin"
    val nullObj: Any? = null
    println(kotlinStr + nullObj) // Prints: Kotlinnull
}

Source