closure--examples/questions

1.

// interviewer: what will the following code output?
const arr = [10, 12, 15, 21];
for (var i = 0; i < arr.length; i++) {
  setTimeout(function() {
    console.log('Index: ' + i + ', element: ' + arr[i]);
  }, 3000);
}

Index: 4, element: undefined(printed 4 times).

2.

function fun(n,o) {
  console.log(o)
  return {
    fun:function(m){
      return fun(m,n);
    }
  };
}
var a = fun(0);  a.fun(1);  a.fun(2);  a.fun(3);//undefined,?,?,?
var b = fun(0).fun(1).fun(2).fun(3);//undefined,?,?,?
var c = fun(0).fun(1);  c.fun(2);  c.fun(3);//undefined,?,?,?

//问:三行a,b,c的输出分别是什么？

//答案：
//a: undefined,0,0,0
//b: undefined,0,1,2
//c: undefined,0,1,1