let variable1 = value1

var l = 10;
{
  let l = 2;
  // At this point, l = 2.
}
// At this point, l = 10.

// Additional ways to declare a variable using let.
let index;
let name = "Thomas Jefferson";
let answer = 42, counter, numpages = 10;
let myarray = new Array();

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

for(let j = 0; j < 10; j++){}
console.log(j); //"ReferenceError: j is not defined

console.log(a); // 输出undefined
console.log(b); // 报错ReferenceError
console.log(c); // 报错ReferenceError
var a = 2;
let b = 2;

typeof x; // ReferenceError
typeof y // undefined
let x;

let x = 1;
let x; // "SyntaxError: Identifier 'x' has already been declared

var y = 2;
var y = 3; // y = 3

// 匿名函数写法
(function () {
 var tmp = ...;
 ...
}());

// 块级作用域写法
{
 let tmp = ...;
 ...
}

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

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

"use strict";
var a = 1;
console.log(window.a) // 1

let b = 1;
console.log(window.b) // undefined