How to use parameters inside a block when defining an extension function?

val test: Int.(String) -> Int = {
  plus((this))
}
        

When defining this type of extension function, how can I use arguments( Here, the argument of type String) inside the block?

When defining extension functions at the same time as declaration like this, can only this be used?

You can access it using it:

val test: Int.(String) -> Int = {
  println("this = $this")
  println("it = $it")
  42
}

fun main() {
    println("result = " + 1.test("a"))
}

This will output

this = 1
it = a
result = 42

An alternative is to introduce a lambda-parameter:

val test: Int.(String) -> Int = { s ->
  println("this = $this")
  println("s = $s")
  42
}

fun main() {
    println("result = " + 1.test("a"))
}

This will output

this = 1
s = a
result = 42