The difference between let and var in JS

1. Differences in scope:

{
  let a = 10;
  var b = 1;
 }
  a // ReferenceError: a is not defined.
  b // 1

• varThe keyword is a global variable or a function variable

• letThe keyword is a block scope

##2. The difference between defining and using variables:

// var 的情况
console.log(foo); // 输出undefined
var foo = 2;
 
// let 的情况
console.log(bar); // 报错ReferenceError
let bar = 2;

• let must be defined first and then used,

• var can be used first and then declared. When using an undefined variable, its value is undefined

##3 .Differences in repeated declarations:

   var a;
   var a;

   
   let b;
   let b;

• var

  The repeated declaration of keywords is correct;

• let

  Repeat keyword declaration error; Uncaught SyntaxError: Identifier 'b' has already been declared

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