The difference between var, let and const in js

This article mainly introduces the differences between var, let, and const in js. It has a certain reference value. Now I share it with everyone. Friends in need can refer to it.

//1.var定义的变量可以修改，如果不初始化会输出undefined，不会报错。
var a;
console.log(a); //undefined


//2.let是块级作用域，函数内部使用let定义后，对函数外部无影响。
let c = 3;
console.log(c)

function change(){
let c = 6;
console.log(c)
}
change();

(1)只要块级作用域于中存在let命令，它所声明的变量就绑定在这个区域中，不再受外部的影响。

var a = 10;
{
  console.log(a);   //undefined   (作用域内部变量不受外部影响，还有就是let不存在变量提升，所以才会报未定义)
  let a = 3;
  console.log(a);    //3
}

(2)let不允许在同一个作用域内，重复声明同一个变量
{
  var a = 2;
  let a  = 2;
  console.log(a)     // Error: Identifier 'a' has already been declared
}


//3.const定义的变量不可以修改，而且必须初始化。
//const b; //这样定义不对，必须赋值初始化
const b=1;

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