Coursera.js

// Good example for observer designer pattern
class Coursera {
  constructor() {
    this.subscribeList = new Map();
    this.subscribeOnceList = new Map();
    this.subscribeOnceAsyncList = new Map();
  }

  subscribe(name, callback) {
    if (!this.subscribeList.has(name)) {
      this.subscribeList.set(name, [callback]);
    } else {
      const callBackList = this.subscribeList.get(name);
      this.subscribeList.set(name, [...callBackList, callback]);
    }
    return () => {
      this.remove(name, callback);
    };
  }

  remove(name, callback) {
    const callBackList = this.subscribeList.get(name);
    const callBackListFiltered = callBackList.filter((callbackList) => {
      callbackList !== callback;
    });
    this.subscribeList.set(name, callBackListFiltered);
  }

  subscribeOnceAsync(name) {
    return new Promise((resolve) => {
      if (!this.subscribeOnceAsyncList.has(name)) {
        this.subscribeOnceAsyncList.set(name, [resolve]);
      } else {
        const callBackList = this.subscribeList.get(name);
        this.subscribeOnceAsyncList.set(name, [...callBackList, resolve]);
      }
    });
  }
}