IIFE (Immediately Invoked Function Expression)

var result = (function outer() {
  var x = 10;
  return (function inner() {
    var y = 20;
    return x + y;
  })();
})();

console.log(result);

10
20
30
The output will be 30. This code demonstrates nested IIFEs with closure. The outer IIFE defines a variable x with a value of 10. It then defines and immediately invokes an inner IIFE. This inner function creates its own variable y with a value of 20, and returns x + y. Because of closure, the inner function has access to variables from its own scope as well as from the enclosing (outer) function scope, so it can access x. The outer IIFE returns the result of the inner IIFE (which is 30), and this value is assigned to the variable 'result'. This pattern of nested IIFEs can be used to create more complex scoping arrangements and data encapsulation, though it's important to balance this with code readability.
Error: x is not defined