extends

class ChildClass extends ParentClass { /* … */ }

ParentClass

An expression that evaluates to a constructor function (including a class) or null.

The extends keyword can be used to subclass custom classes as well as built-in objects.

Any constructor that can be called with new and has the prototype property can be the candidate for the parent class. The two conditions must both hold — for example, bound functions and Proxy can be constructed, but they don't have a prototype property, so they cannot be subclassed.

function OldStyleClass() {
  this.someProperty = 1;
}
OldStyleClass.prototype.someMethod = function () {};

class ChildClass extends OldStyleClass {}

class ModernClass {
  someProperty = 1;
  someMethod() {}
}

class AnotherChildClass extends ModernClass {}

The prototype property of the ParentClass must be an Object or null, but you would rarely worry about this in practice, because a non-object prototype doesn't behave as it should anyway. (It's ignored by the new operator.)

function ParentClass() {}
ParentClass.prototype = 3;

class ChildClass extends ParentClass {}
// Uncaught TypeError: Class extends value does not have valid prototype property 3

console.log(Object.getPrototypeOf(new ParentClass()));
// [Object: null prototype] {}
// Not actually a number!

extends sets the prototype for both ChildClass and ChildClass.prototype.

class ParentClass {}
class ChildClass extends ParentClass {}

// Allows inheritance of static properties
Object.getPrototypeOf(ChildClass) === ParentClass;
// Allows inheritance of instance properties
Object.getPrototypeOf(ChildClass.prototype) === ParentClass.prototype;

The right-hand side of extends does not have to be an identifier. You can use any expression that evaluates to a constructor. This is often useful to create mixins. The this value in the extends expression is the this surrounding the class definition, and referring to the class's name is a ReferenceError because the class is not initialized yet. await and yield work as expected in this expression.

class SomeClass extends class {
  constructor() {
    console.log("Base class");
  }
} {
  constructor() {
    super();
    console.log("Derived class");
  }
}

new SomeClass();
// Base class
// Derived class

While the base class may return anything from its constructor, the derived class must return an object or undefined, or a TypeError will be thrown.

class ParentClass {
  constructor() {
    return 1;
  }
}

console.log(new ParentClass()); // ParentClass {}
// The return value is ignored because it's not an object
// This is consistent with function constructors

class ChildClass extends ParentClass {
  constructor() {
    super();
    return 1;
  }
}

console.log(new ChildClass()); // TypeError: Derived constructors may only return object or undefined

If the parent class constructor returns an object, that object will be used as the this value for the derived class when further initializing class fields. This trick is called "return overriding", which allows a derived class's fields (including private ones) to be defined on unrelated objects.

Here are some things you may expect when extending a class:

• When calling a static factory method (like Promise.resolve() or Array.from()) on a subclass, the returned instance is always an instance of the subclass.
• When calling an instance method that returns a new instance (like Promise.prototype.then() or Array.prototype.map()) on a subclass, the returned instance is always an instance of the subclass.
• Instance methods try to delegate to a minimal set of primitive methods where possible. For example, for a subclass of Promise, overriding then() automatically causes the behavior of catch() to change; or for a subclass of Map, overriding set() automatically causes the behavior of the Map() constructor to change.

However, the above expectations take non-trivial efforts to implement properly.

• The first one requires the static method to read the value of this to get the constructor for constructing the returned instance. This means [p1, p2, p3].map(Promise.resolve) throws an error because the this inside Promise.resolve is undefined. A way to fix this is to fall back to the base class if this is not a constructor, like Array.from() does, but that still means the base class is special-cased.
• The second one requires the instance method to read this.constructor to get the constructor function. However, new this.constructor() may break legacy code, because the constructor property is both writable and configurable and is not protected in any way. Therefore, many copying built-in methods use the constructor's [Symbol.species] property instead (which by default just returns this, the constructor itself). However, [Symbol.species] allows running arbitrary code and creating instances of arbitrary type, which poses a security concern and greatly complicates subclassing semantics.
• The third one leads to visible invocations of custom code, which makes a lot of optimizations harder to implement. For example, if the Map() constructor is called with an iterable of x elements, then it must visibly invoke the set() method x times, instead of just copying the elements into the internal storage.

These problems are not unique to built-in classes. For your own classes, you will likely have to make the same decisions. However, for built-in classes, optimizability and security are a much bigger concern. New built-in methods always construct the base class and call as few custom methods as possible. If you want to subclass built-ins while achieving the above expectations, you need to override all methods that have the default behavior baked into them. Any addition of new methods on the base class may also break the semantics of your subclass because they are inherited by default. Therefore, a better way to extend built-ins is to use composition.

extends null was designed to allow easy creation of objects that do not inherit from Object.prototype. However, due to unsettled decisions about whether super() should be called within the constructor, it's not possible to construct such a class in practice using any constructor implementation that doesn't return an object. The TC39 committee is working on re-enabling this feature.

new (class extends null {})();
// TypeError: Super constructor null of anonymous class is not a constructor

new (class extends null {
  constructor() {}
})();
// ReferenceError: Must call super constructor in derived class before accessing 'this' or returning from derived constructor

new (class extends null {
  constructor() {
    super();
  }
})();
// TypeError: Super constructor null of anonymous class is not a constructor

Instead, you need to explicitly return an instance from the constructor.

class NullClass extends null {
  constructor() {
    // Using new.target allows derived classes to
    // have the correct prototype chain
    return Object.create(new.target.prototype);
  }
}

const proto = Object.getPrototypeOf;
console.log(proto(proto(new NullClass()))); // null

The first example creates a class called Square from a class called Polygon. This example is extracted from this live demo (source).

class Square extends Polygon {
  constructor(length) {
    // Here, it calls the parent class' constructor with lengths
    // provided for the Polygon's width and height
    super(length, length);
    // Note: In derived classes, super() must be called before you
    // can use 'this'. Leaving this out will cause a reference error.
    this.name = "Square";
  }

  get area() {
    return this.height * this.width;
  }
}

Classes cannot extend regular (non-constructible) objects. If you want to inherit from a regular object by making all properties of this object available on inherited instances, you can instead use Object.setPrototypeOf():

const Animal = {
  speak() {
    console.log(`${this.name} makes a noise.`);
  },
};

class Dog {
  constructor(name) {
    this.name = name;
  }
}

Object.setPrototypeOf(Dog.prototype, Animal);

const d = new Dog("Mitzie");
d.speak(); // Mitzie makes a noise.

This example extends the built-in Date object. This example is extracted from this live demo (source).

class MyDate extends Date {
  getFormattedDate() {
    const months = [
      "Jan", "Feb", "Mar", "Apr", "May", "Jun",
      "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
    ];
    return `${this.getDate()}-${months[this.getMonth()]}-${this.getFullYear()}`;
  }
}

All JavaScript objects inherit from Object.prototype by default, so writing extends Object at first glance seems redundant. The only difference from not writing extends at all is that the constructor itself inherits static methods from Object, such as Object.keys(). However, because no Object static method uses the this value, there's still no value in inheriting these static methods.

The Object() constructor special-cases the subclassing scenario. If it's implicitly called via super(), it always initializes a new object with new.target.prototype as its prototype. Any value passed to super() is ignored.

class C extends Object {
  constructor(v) {
    super(v);
  }
}

console.log(new C(1) instanceof Number); // false
console.log(C.keys({ a: 1, b: 2 })); // [ 'a', 'b' ]

Compare this behavior with a custom wrapper that does not special-case subclassing:

function MyObject(v) {
  return new Object(v);
}
class D extends MyObject {
  constructor(v) {
    super(v);
  }
}
console.log(new D(1) instanceof Number); // true

You might want to return Array objects in your derived array class MyArray. The species pattern lets you override default constructors.

For example, when using methods such as Array.prototype.map() that return the default constructor, you want these methods to return a parent Array object, instead of the MyArray object. The Symbol.species symbol lets you do this:

class MyArray extends Array {
  // Overwrite species to the parent Array constructor
  static get [Symbol.species]() {
    return Array;
  }
}

const a = new MyArray(1, 2, 3);
const mapped = a.map((x) => x * x);

console.log(mapped instanceof MyArray); // false
console.log(mapped instanceof Array); // true

This behavior is implemented by many built-in copying methods. For caveats of this feature, see the subclassing built-ins discussion.

Abstract subclasses or mix-ins are templates for classes. A class can only have a single superclass, so multiple inheritance from tooling classes, for example, is not possible. The functionality must be provided by the superclass.

A function with a superclass as input and a subclass extending that superclass as output can be used to implement mix-ins:

const calculatorMixin = (Base) =>
  class extends Base {
    calc() {}
  };

const randomizerMixin = (Base) =>
  class extends Base {
    randomize() {}
  };

A class that uses these mix-ins can then be written like this:

class Foo {}
class Bar extends calculatorMixin(randomizerMixin(Foo)) {}

Inheritance is a very strong coupling relationship in object-oriented programming. It means all behaviors of the base class are inherited by the subclass by default, which may not always be what you want. For example, consider the implementation of a ReadOnlyMap:

class ReadOnlyMap extends Map {
  set() {
    throw new TypeError("A read-only map must be set at construction time.");
  }
}

It turns out that ReadOnlyMap is not constructible, because the Map() constructor calls the instance's set() method.

const m = new ReadOnlyMap([["a", 1]]); // TypeError: A read-only map must be set at construction time.

We may get around this by using a private flag to indicate whether the instance is being constructed. However, a more significant problem with this design is that it breaks the Liskov substitution principle, which states that a subclass should be substitutable for its superclass. If a function expects a Map object, it should be able to use a ReadOnlyMap object as well, which will break here.

Inheritance often leads to the circle-ellipse problem, because neither type perfectly entails the behavior of the other, although they share a lot of common traits. In general, unless there's a very good reason to use inheritance, it's better to use composition instead. Composition means that a class has a reference to an object of another class, and only uses that object as an implementation detail.

class ReadOnlyMap {
  #data;
  constructor(values) {
    this.#data = new Map(values);
  }
  get(key) {
    return this.#data.get(key);
  }
  has(key) {
    return this.#data.has(key);
  }
  get size() {
    return this.#data.size;
  }
  *keys() {
    yield* this.#data.keys();
  }
  *values() {
    yield* this.#data.values();
  }
  *entries() {
    yield* this.#data.entries();
  }
  *[Symbol.iterator]() {
    yield* this.#data[Symbol.iterator]();
  }
}

In this case, the ReadOnlyMap class is not a subclass of Map, but it still implements most of the same methods. This means more code duplication, but it also means that the ReadOnlyMap class is not strongly coupled to the Map class, and does not easily break if the Map class is changed, avoiding the semantic issues of built-in subclassing. For example, if the Map class adds an emplace() method that does not call set(), it would cause the ReadOnlyMap class to no longer be read-only unless the latter is updated accordingly to override emplace() as well. Moreover, ReadOnlyMap objects do not have the set method at all, which is more accurate than throwing an error at runtime.

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