编程题

闭包

function f1() {
  var sum = 0;
  function f2() {
    sum++;
    return f2;
  }
  f2.valueOf = -function () {
    return sum;
  };
  f2.toString = function () {
    return sum + "";
  };
  return f2;
}
console.log(+f1()); //0
console.log(+f1()());//1
console.log(+f1()()());//2