BFE.devでのAI解答例 - JavaScript コーディング問題
57. Observableを作る

以下が求められているObservableの実装例です。

class Observable {  constructor(subscribeFunc) {    this._subscribe = subscribeFunc;  }  subscribe(observerOrNext, error, complete) {    // if observerOrNext is an observer object    if (typeof observerOrNext === 'object') {      return this._subscribe(observerOrNext);    }    // if observerOrNext is a function    return this._subscribe({      next: observerOrNext,      error: error,      complete: complete    });  }  // helper method to create observable from iterable  static from(iterable) {    return new Observable(subscriber => {      for (const item of iterable) {        subscriber.next(item);      }      subscriber.complete();    });  }  // helper method to create observable from a promise  static fromPromise(promise) {    return new Observable(subscriber => {      promise.then(        value => {          subscriber.next(value);          subscriber.complete();        },        reason => subscriber.error(reason)      );    });  }  // helper method to create observable from an event  static fromEvent(target, eventName) {    return new Observable(subscriber => {      const eventHandler = e => subscriber.next(e);      target.addEventListener(eventName, eventHandler);      // return cleanup function      return () => {        target.removeEventListener(eventName, eventHandler);      };    });  }  // helper method to merge multiple observables  static merge(...observables) {    return new Observable(subscriber => {      let activeSubscriptions = observables.length;      const subscriptions = observables.map(obs =>        obs.subscribe({          next: value => subscriber.next(value),          error: error => subscriber.error(error),          complete: () => {            activeSubscriptions--;            if (activeSubscriptions === 0) {              subscriber.complete();            }          }        })      );      // return cleanup function      return () => {        subscriptions.forEach(sub => sub.unsubscribe());      };    });  }}

この実装では、subscribe()メソッドの引数として、Observerオブジェクトか、next/error/completeのいずれかを含む関数を受け取り、両方の型の引数を同じように処理できます。また、from()、fromPromise()、fromEvent()、merge()といったヘルパーメソッドを持っており、Observableインスタンスを作りやすくなっています。