for (var i = 0; i < 10; i++) {
 console.log(i); // 0,1,2...,9
}
console.log(i); //10

if(true){
 var s = 20;
}
console.log(s); //20

 {
  let a = 10;
  var b = 9;
 }
 alert( a ); // ReferenceError: a is not defined
 alert( b ); //9

var a = [];
 for(var i = 0; i < 10; i++) {
  var c = i;
  a[i] = function() {
   console.log( c );
  };
 }
 a[6](); //9

var a = [];
for (var i = 0; i < 10; i++) {
  let c = i;
  a[i] = function() {
   console.log( c );
  };
}
a[6](); //6

 function do_someting() {
  console.log( a ); //ReferenceError
  let a = 2;
 }

 // 报错
 {
  let a = 10;
  var a = 1;
 }

 // 报错
 {
  let a = 10;
  let a = 1;
 }

 function foo() {
  let n = 6; 
  if (true) {
   let n = 10;
  }
  console.log( n ); //6
 }

 function f() { console.log("I am outside"); }
 (function () {
  if (false) {
   //重复声明一次函数f
   function f() {
    console.log("I am inside");
   }
  }

  f();
 }());

 const PI = 3.1415;
 console.log( PI　);　//PI

 PI = 3;
 console.log( PI ); //3.1415

 const PI = 3.1;
 console.log( PI ); //3.1415

 if（condition） {
  const MAX = 5;
 }

 // 常量MAX在此处不可见（或者说不可得）

 var message = "Hello!";
 let age = 25;

 //以下两行都会报错
 const message = "Goodbye!";
 const age = 30;