ES6-const

const也用来声明变量，但是声明的是常量。
一旦声明，常量的值就不能改变
例子：

const Pi = 3.1415926535;
console.log(Pi);
Pi = 3; // Pi is read-only -- Error
console.log(Pi);

const块级作用域：

  if (true) {
    const Pi = 3.14159265;
};
console.log(Pi);    //Pi is not defined

const暂时性死区:

 if (true) {
    console.log(Pi);    //undefined
    const Pi = 3.14159265;
};

不可重复声明：

{
    var a   = 100;
    const a = 200;
    console.log(a);
}

const对象：

const person = {};
person.name  = "Zhangsan";
person.age   = 30;

console.log(person.name);   //Zhangsan
console.log(person.age);    //30
console.log(person);        //Object {name: "Zhangsan", age: 30}


     //如果再次输出person=｛｝结局如下：；
       person = {}; //person is read-only

==========================================================

 const arr = [];
console.log(arr);
console.log(arr.length);
console.log("------");
arr.push("Hello world!");
console.log(arr);
console.log(arr.length);
console.log("------");
arr.length = 0;
console.log(arr);
console.log(arr.length);
console.log("------");

// 再次arr会报错
arr = ["Hello Everyone!"];