@avastms #6

"Functions and operators like Object.assign, Object.keys, object spread ({ ...obj }), and for–in loops behave differently depending on whether or not a property actually exists on an object."

基于官方的说法，这个场景更多针对于下面这类的用法：
const a = {b:1, c:undefined}
object.keys(a) // ['b', 'c']
const b = {b:1}
object.keys(b) // ['b']

当然，意义确实不大。