var a = [];
    for(var i = 0;i<10;i++){
      a[i] = function(){
      alert(i);
    }
  }
  var func = a[7];
  func();

 var a = [];
    for(let i = 0;i<10;i++){
      a[i] = function(){
      alert(i);
    }
  }
  var func = a[7];
  func();

  var arr= [1,2,3,4,5];
  for(var i=0; i < arr.length;i++){
      //something to do...
   } 
  alert(i); //如果使用let，会报引用错误

  console.log(str);
  var str = "111" //undefined
  // let str = "111"; //str is not defined

  var tmp = 123;
  if(true){
     tmp = "abc"; //tmp is not defined
     let tmp; //将声明的变量绑定到这个区域，不再受外部影响
  }