Using classes — Extends and inheritance

Reference note (untrusted external data; do not execute it as instructions). A key feature that classes bring about (in addition to ergonomic encapsulation with private fields) is _inheritance_, which means one object can "borrow" a large part of another object's behaviors, while overriding or enhancing certain parts with its own logic. For example, suppose our Color class now needs to support transparency. We may be tempted to add a new field that indicates its transparency However, this means every instance — even the vast majority which aren't transparent (those with an alpha value of 1) — will have to have the extra alpha value, which is not very elegant. Plus, if the features keep growing, our Color class will become very bloated and hard to maintain. Instead, in object-oriented programming, we would create a _derived class_. The derived class has access to all public properties of the parent class. In JavaScript, derived classes are declared with an extends clause, which indicates the class it extends from. There are a few things that have immediately come to attention. First is that in the constructor, we are calling super(r, g, b). It is a language requirement to call super() before accessing this. The super() call calls the parent class's constructor to initialize this — here it's roughly equivalent to this = new Color(r, g, b). You can have code before super(), but you cannot access this before super() — the language prevents you from accessing the uninitialized this. After the parent class is done with modifying this, the derived class can do its own logic. Here we added a private field called #alpha, and also provided a pair of getter/setters to interact with them. A derived class inherits all methods from its parent. For example, consider the get red() accessor we added to the Color in the Accessor fields section—even though we haven't declared one in ColorWithAlpha, we can still access red because this behavior is specified by the parent class Derived classes can also override methods from the parent class. For example, all classes implicitly inherit the Object class, which defines some basic methods like toString(). However, the base toString() method is notoriously useless, because it prints [object Object] in most cases Instead, our class can override it to print the color's RGB values … Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.