Anonymous function variable problem

var name = 'World!';
(function () {
    console.log('name ' + name);
    if (typeof name === 'undefined') {
      var name = 'Jack';
      console.log('Goodbye ' + name);
    } else {
      console.log('Hello ' + name);
    }
})();

name undefined
Goodbye Jack

 

(1).JavaScript The special thing about variables is , You can reference variables declared later without throwing an exception . This concept is called variable declaration promotion (hoisting);

(2).JavaScript Variables feel like being “ promote ” Or moved to the top of a function or statement . However, the promoted variable will return  undefined value .

(3). Declare and initialize after using or referencing a variable , This promoted reference will still get  undefined value .

(4). here Example 3 in Test01 Is the key to this problem ,Test01 and Test02 It's very similar ( At least I look similar ). But the final output cannot be （1）、（2）、（3） To explain

　　 After checking the official API Document discovery , In anonymous function (function() {})(); A closure is formed in the use of （ a key , If you are not clear, you can refer to the concept of closures , The concept of closure is easy to understand ）;

　　 Due to the existence of closures , Global variables are naturally inaccessible （ An important reason for closures is to avoid accessing global variables ）, so Test01 function The output of the first sentence in is undefined.

Summarize the reasons ： Anonymous functions (function() {})(); Is a special way to write closures .