ECMAScript 6中“扩展”原型的语法

Question

Since classes in ECMAScript 6 is simply syntactical sugar over JavaScript's existing prototype-based inheritance, it is still possible to "extend" (for lack of a better word) the prototype of an existing class with EMCAScript 5 syntax and add methods to it even after the class declaration in a completely separate file.

/* Dog.js */

class Dog {
    woof() {
        console.log('woof woof');
    }
}

module.exports = Dog;

/* main.js */

var Dog = require('./Dog');

Dog.prototype.bark = function() {
    console.log('bark bark');
};

var dog = new Dog();
dog.woof(); // 'woof woof'
dog.bark(); // 'bark bark'

Is there a way to achieve the same functionality as above, but maintain the ECMAScript 6 class syntax in main.js as opposed to going back to the prototype syntax? Trying something such as class Dog extends Dog throws an error as the Dog class is already declared.

Note that this is not extending in the sense of inheritance, as that would require creating a new class rather than adding methods to an existing class.

class Dog {
    woof() {
        console.log('woof woof');
    }
}

class AdvancedDog extends Dog {
    bark() {
        console.log('bark bark');
    }
}

var dog = new Dog();
dog.woof(); // 'woof woof'
dog.bark(); // Error, not what is needed.

Answer 1

No, there's no way of doing what you want to achieve.

A class can't be partially declared across many files.

BTW, you should be able to use some task runner like Grunt or Gulp and implement a task to merge many files declaring the same file that could generate a single file with a single class declaration from the other ones.

Probably this could be also implemented as a Babel plugin.

Answer 2

The semantics of extends is prototypical inheritance, while the wanted behaviour is prototype modification. It is not an everyday task and it is not something that has special syntax in ES6.

It is possible to do this with

Object.assign(Dog.prototype, {
    bark() {
        console.log('bark bark');
    }
});

Notice that bark method in object literal is enumerable, while ES6 class methods are non-enumerable . If this trait is crucial, Object.defineProperty should be additionally used.