Async websocket calls with ES6

gistfile1.js

const marker = () => {
  return Math.random()
    .toString(36)
    .slice(2)
    .padStart(10, "0")
}

export default class AsyncSocket {
  constructor(url = null, config = {}) {
    this.url = url
    this.config = config
    this.listeners = []
    this.promises = []
    this.server = null
    this.waitFor = this.waitFor.bind(this)
    this.resolveMessages = this.resolveMessages.bind(this)
    this.sendAsync = this.sendAsync.bind(this)
  }

  listen(callback) {
    if (!this.server) console.warn("No server found")
    this.listeners.push(callback)
  }

  send(payload) {
    let data = JSON.stringify(payload)
    if (this.server.readyState === 1) this.server.send(data)
  }

  sendAsync(payload) {
    if (!"postback" in payload) {
      console.warn("Please provide postback for async calls")
      return
    }
    return new Promise(resolve => {
      this.server.send(JSON.stringify(payload))
      this.promises.push({ type: "wait_for_postback", action: payload.postback.action, promise: resolve, marker: marker() })
    })
  }

  waitFor(action) {
    return new Promise(resolve => {
      this.promises.push({ type: "wait_for_postback", action, promise: resolve, marker: marker() })
    })
  }

  resolveMessages(message) {
    const data = JSON.parse(message.data)
    this.listeners.forEach(x => x(data))
    this.promises.forEach(x => {
      if (data.postback && data.postback.action === x.action && x.type === "wait_for_postback") {
        x.promise(data)
        this.promises = this.promises.filter(i => i.marker !== x.marker)
      }
    })
  }

  close() {
    if (this.server) this.server.close()
  }

  async open() {
    return new Promise((resolve, reject) => {
      this.server = new WebSocket(this.url)
      this.server.onmessage = message => this.resolveMessages(message)
      this.server.onopen = () => resolve(this)
      this.server.onerror = err => reject(err)
    })
  }
}