IIFE · jsrosetta

An IIFE (Immediately Invoked Function Expression) is a function defined and called right away, usually to create its own scope. Go, Rust, and Swift have no special syntax for this — you just define an anonymous function or closure and call it immediately. Java is harder: a lambda expression needs a concrete functional-interface type, so it can't be called directly — you have to cast it first. Nobody writes real Java like this; a plain { ... } block or a private method is the usual way to create a scope instead of faking an IIFE.

Defining and calling a function immediately

(function (name) {
  console.log('hello', name);
})('bob'); // hello bob

package main
 
import "fmt"
 
func main() {
	func(name string) {
		fmt.Println("hello", name)
	}("bob") // hello bob
}

fn main() {
    (|name: &str| {
        println!("hello {name}");
    })("bob"); // hello bob
}

({ (name: String) in
    print("hello \(name)")
})("bob") // hello bob

void main() {
    // a lambda needs a concrete functional interface, so it has to be cast
    // before it can be called right away — rarely written this way in real Java
    ((Consumer<String>) name -> IO.println("hello " + name)).accept("bob"); // hello bob
}