{
let a = 10;
var b = 1;
}
a // ReferenceError: a is not defined.
b // 1

var tmp = 123;
if (true) {
tmp = 'abc'; // ReferenceError
let tmp;
}

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

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();
}());
//I am inside! ES5 函数提升
//I am outside! ES6 块级作用域

var a = 1;
// 如果在Node的REPL环境，可以写成global.a
// 或者采用通用方法，写成this.a
window.a // 1
let b = 1;
window.b // undefined

const foo='foo';
foo='bar';//TypeError: Assignment to constant variable.

const foo=[];　　
foo=[1];//Assignment to constant variable.

const foo=[1,2,3];
foo[1]=4;
console.log(foo)//[1, 4, 3]

const foo=Object.freeze([1,2,3]);
foo[1]=4;
console.log(foo)//[1, 2, 3]