Let 与 Var 关键字的区别

function varGreeter(){
  var a = 10;        
  var a = 20; //a is replaced
  console.log(a);
}
varGreeter();

function varGreeter(){
  let a = 10;        
 let a = 20; //SyntaxError: 
 //Identifier 'a' has already been declared
  console.log(a);
}
varGreeter();

{
  console.log(c); // undefined. 
  //Due to hoisting
  var c = 2;
}

{
  console.log(b); // ReferenceError: 
  //b is not defined
  let b = 3;
}