Closures and IIFE in JavaScript

As you already know, you can call an anonymous function in-place and assign the result of the work to some variable:

javascriptlet result = function() {
  return '!';
}();

console.log(result); // shows '!'

Often the immediately invoked function is then enclosed in parentheses, although this is not necessary. This is done so that it is immediately clear that the function is called in-place:

javascriptlet result = (function() {
  return '!';
})();

console.log(result); // shows '!'

Let now the result of the function called in-place be an anonymous function:

javascriptlet func = (function() {
  return function() {
    console.log('!');
  }
})();

func(); // shows '!'

Let's make the returned function store the variable num in the closure:

javascriptlet func = (function() {
  let num = 1;
  
  return function() {
    console.log(num);
  }
})();

If you add num++, then the function func will become a ready-made counter:

javascriptlet func = (function() {
  let num = 1;
  
  return function() {
    console.log(num);
    num++;
  }
})();

func(); //shows 1
func(); //shows 2
func(); //shows 3
func(); //shows 4
func(); //shows 5