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.