micro-subject: provide basic subscribe/notify/unsubscribe functionality in a dozen or so lines of code

micro-subject.js

/**
 * Provide basic subscribe/notify/unsubscribe functionality in a
 * dozen or so lines of code.
 */

/**
 * @template T
 * @typedef {(value?: T) => void} ObserverFn
 */

/**
 * @typedef {() => void} UnsubscribeFn
 */

/**
 * @template T
 * @typedef {Object} Subject<T>
 * @property {(observer: ObserverFn<T>) => UnsubscribeFn} subscribe
 * @property {(value?: T) => void} notify
 */

/**
 * @template T
 */
const subject = () => {
  /**
   * Store observers.
   *
   * @type {Map<number, ObserverFn<T>>}
   */
  const map = new Map()

  // Track subscription id.
  let nextId = 0

  /**
   * @param {number} id
   *
   * @returns {void}
   */
  const unsubscribe = (id) => void map.delete(id)

  /**
   * @type {Subject<T>}
   */
  const _subject = {
    /**
     * @param {ObserverFn<T>} observer
     *
     * @return {UnsubscribeFn}
     */
    subscribe(observer) {
      if (typeof observer !== 'function') {
        throw new Error('<subject.subscribe> observer must be a function')
      }

      const id = nextId++
      map.set(id, observer)
      return unsubscribe.bind(null, id)
    },

    /**
     * @param {T} [value]
     *
     * @returns {void}
     */
    notify(value) {
      for (const observer of map.values()) {
        observer(value)
      }
    },
  }

  return _subject
}

export default subject