{
 let a = 10;
 var b = 1;
}

a // ReferenceError: a is not defined.
b // 1

for (let i = 0; i < 10; i++) {}

console.log(i);
//ReferenceError: i is not defined

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

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

console.log(foo); // 输出undefined
console.log(bar); // 报错ReferenceError

var foo = 2;
let bar = 2;

var tmp = 123;

if (true) {
 tmp = 'abc'; // ReferenceError
 let tmp;
}

if (true) {
 // TDZ开始
 tmp = 'abc'; // ReferenceError
 console.log(tmp); // ReferenceError

 let tmp; // TDZ结束
 console.log(tmp); // undefined

 tmp = 123;
 console.log(tmp); // 123
}

typeof x; // ReferenceError
let x;

typeof undeclared_variable // "undefined"

function bar(x = y, y = 2) {
 return [x, y];
}

bar(); // 报错

function bar(x = 2, y = x) {
 return [x, y];
}
bar(); // [2, 2]

// 报错
function () {
 let a = 10;
 var a = 1;
}

// 报错
function () {
 let a = 10;
 let a = 1;
}

function func(arg) {
 let arg; // 报错
}

function func(arg) {
 {
  let arg; // 不报错
 }
}

var tmp = new Date();

function f() {
 console.log(tmp);
 if (false) {
  var tmp = "hello world";
 }
}

f(); // undefined

var s = 'hello';

for (var i = 0; i < s.length; i++) {
 console.log(s[i]);
}

console.log(i); // 5

function f1() {
 let n = 5;
 if (true) {
  let n = 10;
 }
 console.log(n); // 5
}

{{{{{let insane = 'Hello World'}}}}};

{{{{
 {let insane = 'Hello World'}
 console.log(insane); // 报错
}}}};

{{{{
 let insane = 'Hello World';
 {let insane = 'Hello World'}
}}}};

// IIFE 写法
(function () {
 var tmp = ...;
 ...
}());

// 块级作用域写法
{
 let tmp = ...;
 ...
}

// 情况一
if (true) {
 function f() {}
}

// 情况二
try {
 function f() {}
} catch(e) {
}

// ES5严格模式
'use strict';
if (true) {
 function f() {}
}
// 报错

// ES6严格模式
'use strict';
if (true) {
 function f() {}
}
// 不报错

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();
}());

// ES5版本
function f() { console.log('I am outside!'); }
(function () {
 function f() { console.log('I am inside!'); }
 if (false) {
 }
 f();
}());

// ES6版本
function f() { console.log('I am outside!'); }
(function () {
 f();
}());

// ES6的浏览器环境
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();
}());
// Uncaught TypeError: f is not a function

// ES6的浏览器环境
function f() { console.log('I am outside!'); }
(function () {
 var f = undefined;
 if (false) {
  function f() { console.log('I am inside!'); }
 }

 f();
}());
// Uncaught TypeError: f is not a function

// 函数声明语句
{
 let a = 'secret';
 function f() {
  return a;
 }
}

// 函数表达式
{
 let a = 'secret';
 let f = function () {
  return a;
 };
}

// 不报错
'use strict';
if (true) {
 function f() {}
}

// 报错
'use strict';
if (true)
 function f() {}

{
 let t = f();
 t = t * t + 1;
}

let x = do {
 let t = f();
 t * t + 1;
};

let me = 'go'; //globally scoped 
var i = 'able'; //globally scoped 

function ingWithinEstablishedParameters() { 
  let terOfRecommendation = 'awesome worker!'; //function block scoped 
  var sityCheerleading = 'go!'; //function block scoped 
}; 

function allyIlliterate() { 
  //tuce is *not* visible out here 
 
  for( let tuce = 0; tuce < 5; tuce++ ) { 
    //tuce is only visible in here (and in the for() parentheses) 
  }; 
 
  //tuce is *not* visible out here 
}; 
 
function byE40() { 
  //nish *is* visible out here 
 
  for( var nish = 0; nish < 5; nish++ ) { 
    //nish is visible to the whole function 
  }; 
 
  //nish *is* visible out here 
};