EventIterator raw JS file translated from https://github.com/rolftimmermans/event-iterator

EventIterator.js

const noop = () => {}

export default class EventIterator {
  #listen
  #options

  constructor(listen, options) {
    this.#listen = listen
    this.#options = {
      highWaterMark: 100,
      ...options,
    }
    Object.freeze(this)
  }

  [Symbol.asyncIterator]() {
    let pullQueue = []

    const pushQueue = []
    const listen = this.#listen

    let remove = noop
    let finaliser = 0

    const push = value => {
      const resolution = {
        value,
        done: false,
      }

      if (pullQueue.length)
        pullQueue.shift()?.resolve(resolution)
      else {
        pushQueue.push(Promise.resolve(resolution))

        const { highWaterMark } = this.#options

        if (
          highWaterMark !== undefined
          && pushQueue.length >= highWaterMark
        )
          console.log(`EventIterator queue reached ${ pushQueue.length } items`)
      }
    }
    const stop = () => {
      Promise.resolve().then(() => {
        remove()

        finaliser = {
          value: undefined,
          done : true,
        }
        if (pullQueue.length) {
          for (const placeholder of pullQueue)
            placeholder.resolve(finaliser)
          pullQueue = []
        } else pushQueue.push(Promise.resolve(finaliser))
      })
    }
    const fail = error => {
      Promise.resolve().then(() => {
        remove()

        if (pullQueue.length) {
          for (const placeholder of pullQueue)
            placeholder.reject(error)

          pullQueue = []
        } else {
          const rejection = Promise.reject(error)

          // Attach error handler
          // to avoid leaking an unhandled promise rejection.
          rejection.catch(() => {})
          pushQueue.push(rejection)
        }
      })
    }

    remove
      = listen({
        push,
        stop,
        fail,
      }) || remove

    return {
      async next() {
        if (finaliser) return finaliser
        else if (pushQueue.length) return pushQueue.shift()
        return new Promise((resolve, reject) => {
          pullQueue.push({
            resolve,
            reject,
          })
        })
      },

      async return() {
        remove()

        finaliser = {
          value: undefined,
          done : true,
        }
        return finaliser
      },
    }
  }
}

package.json

{
  "name": "event-iterator",
  "version": "0.1.0",
  "type": "module",
  "main": "EventIterator.js"
}