Delete

delete can be used to remove a property from an object. It will remove a property from the
object if it has one. It will not look further in the prototype chain.
Removing a property from an object may allow a property from the prototype chain to shine through:

var adult = {age:26},
    child = Object.create(adult);
    child.age = 8;
delete child.age;
 /* Remove age property from child, revealing the age of the prototype, because then it is not overriden. */
var prototypeAge = child.age;
 // 26, because child does not have its own age property.