Computed value with Observable chains

computed_value.js

class Observable {
    constructor(val) {
        this._value = val;
        this._listeners = [];
    }

    get value() {
        return this._value;
    }

    set value(val) {
        if (this._value !== val) {
            this._value = val;
            this.notify();
        }
    }

    notify() {
        this._listeners.forEach(listener => listener(this._value));
    }

    subscribe(listener) {
        this._listeners.push(listener);
    }
}

class Computed extends Observable {
    constructor(computeFn, observables) {
        super('');
        // Compute initial value
        this._value = this._getComputedVal(computeFn, observables);
        const listener = () => {
            // Compute latest value
            this._value = this._getComputedVal(computeFn, observables);
            this.notify();
        };
        observables.forEach(observable => observable.subscribe(listener));
    }

    get value() {
        return this._value;
    }

    set value(val) {
        throw 'Cannot set computed value';
    }

    _getComputedVal(computeFn, observables) {
        const values = observables.map(observable => observable.value);
        return computeFn(...values);
    }
}

const first = new Observable('John');
const last = new Observable('Doe');

const fullname = new Computed((firstVal, lastVal) => {
    return `${firstVal} ${lastVal}`.trim();
}, [first, last]);
console.log(fullname.value);    // John Doe

first.value = 'Jane';
console.log(fullname.value);    // Jane Doe