1.let,const在代码块内有效，var是在全局范围内有效
{
let test1 = 0;
var test2 = 1;
}
test1 // test1is not defined
test2 // 1

2.let 和const只能声明一次，var 可以声明多次
let test1 = 1;
let test1 = 2;
var test2 = 3;
var test2 = 4;
test1 // Identifier 'test1 ’ has already been declared
test2 // 4

3.var声明变量存在变量提升，let和const不存在变量提升
console.log(a); //ReferenceError: a is not defined
let a = “apple”;

console.log(b); //undefined
var b = “banana”;

console.log©; //ReferenceError: c is not defined
const c = “cstrawberry”;

4.const 声明一个只读变量，声明之后不允许改变。意味着，一旦声明必须初始化，否则会报错。

const PI = “3.1415926”;
PI // 3.1415926

const MY_AGE; // SyntaxError: Missing initializer in const declaration

以上均为学习总结，欢迎补充