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let
const
var

var let . let var, var let .

var message = ".";
alert(message); // .

var let const . var , var .

. var let var . .

var .

var . .

if (true) {
  var test = true; // 'let'  'var' .
}

alert(test); // true(if       )

var test . .

var test let test , test if .

if (true) {
  let test = true; // 'let'  
}

alert(test); // Error: test is not defined

. var .

          for (var i = 0; i < 10; i++) {
  // ...
}

alert(i); // 10,   'i'     .
        

, var .

function sayHi() {
  if (true) {
    var phrase = "Hello";
  }

  alert(phrase); //  .
}

sayHi();
alert(phrase); // Error: phrase is not defined

, var if, for . . var .

var

let .

let user;
let user; // SyntaxError: 'user' has already been declared

var . var .

var user = "Pete";

var user = "John"; //  "var"   (  ).
// ...   .

alert(user); // John

var

var . .

var (, ).

function sayHi() {
  phrase = "Hello";

  alert(phrase);

  var phrase;
}
sayHi();

var phrase .

function sayHi() {
  var phrase;

  phrase = "Hello";

  alert(phrase);
}
sayHi();

, .

function sayHi() {
  phrase = "Hello"; // (*)

  if (false) {
    var phrase;
  }

  alert(phrase);
}
sayHi();

'(hoisting)' . var (hoisted) (hoist . ).

if (false) , . if var sayHi (*) phrase .

function sayHi() {
  alert(phrase);

  var phrase = "Hello";
}

sayHi();

var phrase = "Hello" .

(var)
(=)

() . .

function sayHi() {
  var phrase; //     .

  alert(phrase); // undefined

  phrase = "Hello"; //        .
}

sayHi();

var , var . undefined.

alert phrase . undefined .

var . var . var . ' (immediately-invoked function expressions)'. IIFE .

. .

IIFE .

(function() {

  let message = "Hello";

  alert(message); // Hello

})();

, . .

, (function {}) . . 'function . . .

//     
function() { // <-- Error: Function statements require a function name

  let message = "Hello";

  alert(message); // Hello

}();

" ?" . .

//       .
function go() {

}(); // <--       .

. , .

, .

// IIFE  

(function() {
  alert("  ");
})();

(function() {
  alert("  ");
}());

!function() {
  alert("   NOT  ");
}();

+function() {
  alert("     ");
}();

. .

var let const .

var .
var ( ) .

, .

var . let . let const .