Observable map function

gistfile1.txt

class Observable {

    map(fn) {

        if (typeof fn !== "function")
            throw new TypeError(fn + " is not a function");

        let C = this.constructor[Symbol.species];

        return new C(observer => {
            this[Symbol.observer]({
                start(subscription) {
                    this.subscription = subscription;
                    observer.start(subscription);
                },
                next(value) {
                    try { value = fn(value) }
                    catch (e) {
                        this.subscription.dispose();
                        return observer.error(e) 
                    }

                    return observer.next(value);
                },

                error(value) { return observer.error(value) },
                complete(value) { return observer.complete(value) },
            });
        });
    }
    
}