Observable Array (POC, very slow)

oarray.js

const mediator = new WeakMap();
const notify = (instance, ev, index, item) => mediator.get(instance)[ev].forEach(listener => listener({index, item}));
class OArray extends Array {
    constructor() {
        super();
        mediator.set(this, {changed: [], added: [], removed: []});
    }
    push(...items) {
        for (let i = 0, tl = this.length, l = items.length; i < l; i++) {
            let item = items[i];
            Object.defineProperty(this, tl + i, {get: () => item, set: val => {item = val; notify(this, "changed", tl + i, item);}});
            notify(this, "added", tl + i, item);
        }
    }
    on(ev, listener) {
        let handlers = mediator.get(this);
        if (!handlers[ev]) { throw Error(`${ev} is not registered event`); }
        handlers[ev].push(listener);
    }
}