Changes to the Object prototype object are seen by all objects through prototype chaining, unless the properties and methods subject to those changes are overridden further along the prototype chain. This provides a very powerful although potentially dangerous mechanism to override or extend object behavior.
Date objects inherit from Date.prototype. Array objects inherit from Array.prototype. Person objects inherit from Person.prototype.
The Object.prototype is on the top of the prototype inheritance chain:
Date objects, Array objects, and Person objects inherit from Object.prototype.
Adding Properties and Methods to Objects
Sometimes you want to add new properties (or methods) to all existing objects of a given type.
Sometimes you want to add new properties (or methods) to an object constructor.
Using the prototype Property