When do I need to use hasOwnProperty()?

Question

I read that we should always use hasOwnProperty when looping an object, because the object can be modified by something else to include some keys we don\'t want.

But

Answer 1

When you're using for (var key in obj) it will loop through the given object + its parent objects' properties on the prototype chain until it reaches the end of the chain. As you want to check only a specific object's properties, you need to use hasOwnProperty.

This is not needed in for (var i = 0; i < length; i++) or data.forEach().