Что в js означает (function() { })();? [дубликат]

См. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types#Variable_scope , а именно:

Variable hoisting Another unusual thing about variables in JavaScript is that you can refer to a variable declared later, without getting an exception. This concept is known as hoisting; variables in JavaScript are in a sense "hoisted" or lifted to the top of the function or statement. However, variables that are hoisted will return a value of undefined. So even if you declare and initialize after you use or refer to this variable, it will still return undefined.

/**
 * Example 1
 */
console.log(x === undefined); // true
var x = 3;
/**
 * Example 2
 */
// will return a value of undefined
var myvar = 'my value';
(function() {
  console.log(myvar); // undefined
  var myvar = 'local value';
})();