{
  let a = 1;
  var b = 2;
}
console.log(b); // 2
console.log(a); // a is not defind

var a = [];
//函数和变量i会最先进行声明，同时全局变量i经过for循环赋值为10
for (var i = 0; i < 10; i++) {
  a[i] = function () {
    console.log(i);
  };
}
console.log(i);//10
a[6]();//10

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

console.log(a); // a is not defined
let a = 1;

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

const PI = 3.1415;
console.log(PI); // 3.1415
//PI = 3; // Assignment to constant variable.(不能给常量赋值)
//const PI = 3.1;// Identifier 'PI' has already been declared