js面试题

<script type="text/javascript">
    var a=[];
    for(var i=0;i<10;i++){
        a[i]=function(){
            console.log(i)
        }
    }
    a[6]();//10

    for(let i=0;i<10;i++){
        a[i]=function(){
            console.log(i)
        }
    }
    a[6]();//6
------------------------------------------
    setTimeout(function(){
        console.log(1)
    });
    new Promise(function(resolve,reject){
        console.log(2);
        resolve(3);
    }).then(function(val){
        console.log(val);
    })
    console.log(4);  //2 4 3 1
------------------------------------------
    function fun(n,o){
        console.log(o);
        return {
            fun:function(m){
                return fun(m,n);
            }
        }
    }
      var a=fun(0);  //undefined
      a.fun(1);//fun(0,und).fun(1)  0
      a.fun(2);//0
      a.fun(3);//0
     var b=fun(0).fun(1).fun(2).fun(3); //undefined 0 1 2
     var c=fun(0).fun(1);  //undefined 0
     c.fun(2);//1
     c.fun(3);//1