Update the q 13

Author: yeungon

README.md

@@ -492,6 +492,10 @@ First we have an empty object `x`, then we add another property `hi` for x with
 
 Then we overwrite the property `hi` with a new value 11. Last we have 11 + 1 = 12. `x` has one property and `x.hi` returns 11.
 
+Updated (July 27th 2021). If you write `Object.prototype.hi = 11;` instead of `Object.prototype.hi = ++x.hi;` as written in the code above, then `Object.keys(x)` will return an empty array as `Object.keys(object)` only returns the property of the object itself, not the inherited ones. It means the final result will be 11 rather than 12. For some reason, the code ``Object.prototype.hi = ++x.hi;` will create a property for the object `x` itself and then `Object.keys(x)` gives us the array `["hi"]`. 
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+Yet, if you run `console.log(x.hasOwnProperty("hi"))` it still returns `false`. By the way, when you deliberately add a property for x such as `x.test = "testing"`, then `console.log(x.hasOwnProperty("test"))` returns `true`.
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