Observable.js

function Observable(initialValue, deep = true) {
    const listeners = new Set();
    let value;

    function _observe(newValue, methodNames) {
        value = new Proxy(newValue, {
            get(target, property, receiver) {
                const val = Reflect.get(target, property, receiver);
                if (typeof val === "function" && methodNames.includes(property)) {
                    return function (...args) {
                        const retVal = val.apply(target, args);
                        listeners.forEach(callback => callback(value));
                        return retVal;
                    };
                }
                return val;
            },
            set(target, property, newPropertyValue) {
                if (target[property] !== newPropertyValue) {
                    target[property] = newPropertyValue;
                    listeners.forEach(listener => listener(value));
                }
                return true;
            }
        });
    }

    function _setValue(newValue) {
        if (value === newValue) return;
        if(!deep) {
            value = newValue;
        } else {
            if (newValue?.constructor === Object) { // isPlainObject
                _observe(newValue, []);
            } else if (Array.isArray(newValue)) { // isArray
                _observe(newValue, ["push", "sort", "unshift", "pop", "shift", "splice", "reverse", "fill", "copyWithin"]);
            } else if (newValue instanceof Date) { // isDate
                _observe(newValue, ["setDate", "setFullYear", "setHours", "setMilliseconds", "setMinutes", "setMonth", "setSeconds", "setTime", "setUTCDate", "setUTCFullYear", "setUTCHours", "setUTCMilliseconds", "setUTCMinutes", "setUTCMonth", "setUTCSeconds"]);
            } else if (newValue instanceof HTMLElement) {
                _observe(newValue, ["appendChild", "remove", "replaceWith", "setAttribute", "removeAttribute"]);
            } else if (newValue !== Object(newValue)) { // isPrimitive
                value = newValue;
            } else {
                throw new Error("Observable does not support type: " + typeof newValue);
            }
        }
        listeners.forEach(listener => listener(value));
    }

    _setValue(initialValue);

    return {
        isObservable: true,
        get value() {
            if(Array.isArray(Observable.observablesCalled) && !Observable.observablesCalled.includes(this)) {
                Observable.observablesCalled.push(this);
            }
            return value;
        },
        set value(newValue) {
            _setValue(newValue);
        },
        set(newValue) {
            _setValue(newValue);
        },
        subscribe(listener) {
            listeners.add(listener);
            listener(value);
            return () => listeners.delete(listener);
        },
        unsubscribe(listener) {
            listeners.delete(listener);
        },
        map(mapFunction) {
            const newObservable = Observable();
            const unsubscribe = this.subscribe((unwrappedValue) => {
                if (Array.isArray(unwrappedValue)) {
                    newObservable.value = unwrappedValue.map(mapFunction);
                } else {
                    newObservable.value = mapFunction(unwrappedValue);
                }
            });
            newObservable.dispose = unsubscribe;
            return newObservable;
        }
    };
}

// Returns a function that can be called to get the observables
// that were called during the recording.
Observable.recordObservablesCalled = function() {
    Observable.recorders = Observable.recorders || 0;
    Observable.recorders++;
    Observable.observablesCalled = Observable.observablesCalled || [];
    const currentIndex = Observable.observablesCalled.length;
    return function() {
        const observables = Observable.observablesCalled.slice(currentIndex);
        Observable.recorders--;
        if (Observable.recorders === 0) {
            delete Observable.recorders;
            delete Observable.observablesCalled;
        }
        return observables;
    };
};

module.exports = Observable;