stringSetOf

Creates a new instance of the specialized implementation of HashSet with the specified String elements, which elements the keys as properties of JS object without hashing them.

fun stringSetOf(vararg elements: String): HashSet<String>(source)

fun main() {
    val colors = stringSetOf("red", "green", "blue")
    colors.add("yellow")
    println(colors)                     // [red, green, blue, yellow]
    println(colors.contains("green"))   // true
}

Source