var let const关键字之间的区别及使用场景示例详解

console.log(a); //undefind
var a = 1；
console.log(a); //1

var a;
console.log(a); //undefined
a = 1; 
console.log(a); //1

var carName = "Volvo";
  function person(){
    console.log(carName); // "Volvo"
  } 
  person();

function person() {
    var carName = "Volvo";
    console.log(carName);  // "Volvo"
   }
 person();
 console.log(carName); //报错 Uncaught ReferenceError: a is not defined

var a = 10;
 console.log(a); // 10
  { 
    var a = 2;
    console.log(a); // 2
  }
 console.log(a); // 2

var x = 2;
console.log(x); // 2
var x = 3;
console.log(x); // 3

console.log(x);//报错 Unexpected identifier 'Uncaught'
  let x = 1;
  console.log(x)；// 1

{
      console.log(x); //报错 Unexpected identifier 'Uncaught'
      let x = 1;
      console.log(x);
   }

var x = 2;       // 合法
  let x = 3;       // 不合法
  {
      var x = 4;   // 合法
      let x = 5   // 不合法
  }

let x = 2;       // 合法
let x = 3;       // 不合法
{
    let x = 4;   // 合法
    let x = 5;   // 不合法
}

let x = 2;       // 合法
{
    let x = 3;   // 合法
}
{
    let x = 4;   // 合法
}

{
        console.log(x); //报错 Unexpected identifier 'Uncaught'
        const x = 1;
        console.log(x); // 1
    }

{
        const x = 1;
        console.log(x); //1;
    }
    console.log(x)// 报错 Unexpected identifier 'Uncaught'

const x;
  console.log(x);// Uncaught SyntaxError: Missing initializer in const declaration
  const x = 10;
  x = 20; // 报错，const 变量不可重新赋值

const x = [1, 2, 3];
x.push(4); // 合法，数组本身可以修改
x = [5, 6, 7]; // 报错，变量引用地址不可变