Inheritance in JavaScript comes down to prototypes. Each object has an internal property known as its prototype, which is a link to another object. That object has a prototype of its own. This pattern can repeat until an object is reached that has undefined as its prototype. This is known as the prototype chain, and it's how inheritance works in JavaScript. The following diagram explain the inheritance in JavaScirpt:

When running a search for an object's function definition, JavaScript "walks" the prototype chain until it finds the first definition of a function with the right name. Therefore, overriding it is as simple as providing a new definition on the prototype of the subclass.