The difference between const, let, var

The difference between const, let, var

Similarities : Both can be used to define variables.
Similarities and differences:
1. const and let have block-level scope, but var does not.

eg：
 {
    let a = 10;
  }
  console.log(a)     //（报错：）Uncaught ReferenceError: a is not defined
  //------------------------------------------------------------------
 {
    const a = 10;
  }
  console.log(a)     //（报错：）Uncaught ReferenceError: a is not defined
  //------------------------------------------------------------------
  {
    var a = 10;
  }
  console.log(a)  //10

2. Var promotes variables. const and let do not.
3. The value of const-defined variables cannot generally be modified, but if the assigned value is a reference type, it can be modified because its address has not changed.