19.for in 和 for of

var s = {
  a: 1,
  b: 2,
  c: 3
};
var s1 = Object.create(s);
for (var prop in s1) {
  console.log(prop); //a b c
  console.log(s1[prop]); //1 2 3
}
for (let prop of s1) {
  console.log(prop); //报错如下 Uncaught TypeError: s1 is not iterable
}
for (let prop of Object.keys(s1)) {
  console.log(prop); // a b c
  console.log(s1[prop]); //1 2 3
}