Both of the class has one default and one parameterized constructor.Ĭonsole.WriteLine("Base Class Default Constructor") Ĭonsole.WriteLine("Base Class Parameterized Constructor") Ĭonsole.WriteLine("Derived Class Default Constructor") Ĭonsole.WriteLine("Derived Class Parameterized Constructor") įollowing image shows a quick overview of the inheritance of these two classes.įig : Code Map View of both Derived and Base Class. We use base keyword for this type of initialization.Ĭonsider you have following BaseClass and a DerivedClass. If we derive a class from a base class and want to pass data from the derived class constructor to the constructor of the base class, a derived class constructor is required to call the constructor from its base class. Let’s have a quick understanding on this. to call base class constructor from derived class ? Another frequently asked question in interview for the beginners, and I have seen confusion around answering this. Usage of the following script helps enforce the policy of always calling the parent constructor in the case where default (object) values have been set to properties on the parent "class." Not fool-proof, but should work well to prevent most problems. To demonstrate this, I will define a base class with both a complex and simple property, then define two sub-class instances and update each property respectively: And, since each instance in a set of sub-classes points to the same Prototype instance (see diagram above), updating a complex property updates all sub-class instances (not just the one in question). When you then update a key within that object reference, you are altering the Prototype itself. But, because the object is pass-by-reference, it doesn't return a copy of it rather, it returns a reference to the original object contained in the Prototype. When you read a complex object property, Javascript climbs the prototype chain looking for it as normal. While this is an amazing architecture for memory conservation, we run into problems when we read and update complex pass-by-reference properties defined in our prototype chain. In doing this, Javascript allows each sub-class to leverage the default values defined in its prototype chain while maintaining a separate, unique set of instance values when it updates properties. But, when you write a property to an object, Javascript always writes it directly to the sub-class (not back into the object prototype). See, when you read a property from an object, it travels up the prototype chain looking for it (see diagram above). This is the part I didn't understand at all until I started reading Pro Javascript Design Patterns. ![]() Simple pass-by-value properties, such as strings and numbers never cause a problem because of the asymmetrical reading and writing of properties in the prototype chain. And of this 5%, really, the only tricky part involves complex property values. ![]() It's the other 5%, the property-based inheritence, that can really trip us up if we don't fully understand prototypal inheritence. Normally, this doesn't matter to us because 95% of inheritence is method-based meaning, we use inheritence to factor out common methods into base classes so that they don't have to be redefined in all of our sub-classes. In prototypal inheritence, on the other hand, for each set of sub-classes, there is but one single prototype instance and each sub-class merely gets a pointer to that prototype (base class). Behind the scenes, what happens is that when you access a property on the final class, Javascript will keep moving up the prototype chain in search of the requested property until it finds it (in which case it returns the value) or until it hits a null prototype, in which case it returns undefined (or throws an error).Īt first glance, this might seem like inheritence as you know it in other languages but, there is one huge caveat! In typical inheritence, each sub-class gets a copy of its base class. When I say "appears", I mean that while it seems as though the final class has all of these properties, it does not actually contain them. As you can hopefully see from the graphic, the final class, as seen by the outside world, appears to have all of the properties defined within the entire protoype chain.
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