ES6 class mixin

class_merge.js

(function () {
	'use strict';

	function mergeClass(receivingClass, givingPrototype) {
		if (givingPrototype === Object.prototype) {
			return;
		}

		// fetch inherited properties
		const givingPrototypeProto = Object.getPrototypeOf(givingPrototype);
		if (givingPrototype != null) {
			mergeClass(receivingClass, givingPrototypeProto);
		}
		// done

		const receivingPrototype = receivingClass.prototype;
		for (let propertyName of Object.getOwnPropertyNames(givingPrototype)) {
			if (propertyName === 'contructor') {
				continue;
			}

			receivingPrototype[propertyName] = givingPrototype[propertyName];
		}
	}

	function merge(receivingClass, ...mixins) {
		for (let i = 0; i < mixins.length; i++) {
			const mixin = mixins[i];
			if (mixin == null) {
				throw new Error(`Invalid mixin n° ${i}`);
			}

			if (typeof mixin === 'object') {
				mergeClass(receivingClass, mixin);
			}

			if (typeof mixin === 'function') {
				mergeClass(receivingClass, mixin.prototype);
			}
		}

		return receivingClass;
	}

	// tests

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

		eat() {
			console.log(`${this.name} is now eating.`);
		}

		die() {
			throw new Error('you\'re a terrible person.');
		}
	}

	// supports other classes
	class FlyMixin {
		fly() {
			console.log(`${this.name} is now flying.`);
		}
	}

	// or plain objects
	const WalkMixin = {
		walk() {
			console.log(`${this.name} is now walking.`);
		}
	};

	const Cat = merge(class extends Animal {}, WalkMixin);
	const Bird = merge(class extends Animal {}, WalkMixin, FlyMixin);

	let bird = new Bird('birdy');
	let cat = new Cat('cat... ty ?');

	bird.eat(); // birdy is now eating.
	cat.eat(); // cat... ty ? is now eating.

	bird.walk(); // birdy is now walking.
	cat.walk(); // cat... ty ? is now walking.

	bird.fly(); // birdy is now flying.
	cat.fly(); // Uncaught TypeError: cat.fly is not a function
})();