🕺Async generators are really cool! 🕺

Generator.Event.js

// https://github.com/tc39/proposal-async-iteration

var Read = class Read {
  constructor() {
    this.promise = new Promise((resolve, reject) => {
      this.resolve = resolve
      this.reject = reject
    })
  }
  return(value) {
    this.value = value
    this.done = false
    this.resolve(this)
  }
  cancel() {
    this.value = undefined
    this.done = true
  }
  throw(error) {
    this.error = error
    this.reject(error)
  }
}

var On = class On {
  constructor(type, target, capture=false) {
    this.type = type
    this.target = target
    this.status = 'ready'
    this.readQueue = []
  }
  [Symbol.asyncIterator]() {
    return this
  }
  next() {
    switch (this.status) {
      case "ready": {
        const read = new Read()
        this.readQueue.push(read)
        this.target.addEventListener(this.type, this, this.capture)
        this.status = 'subscribed'
        return read.promise
      }
      case "subscribed": {
        const read = new Read()
        this.readQueue.push(read)
        return read.promise
      }
      case "unsubscribed": {
        return {done:true, value:null}
      }
    }
  }
  handleEvent(event) {
    const read = this.readQueue.shift()
    if (read) {
      read.return(event)
    }
  }
  return() {
    this.status = "unsubscribed"
    this.target.removeEventListener(this.type, this, this.capture)
    
    for (const read of this.readQueue.splice(0)) {
      read.cancel()
    }
  }
  throw(error) {
    this.status = "unsubscribed"
    this.target.removeEventListener(this.type, this.target, this.capture)
    
    for (const read of this.readQueue.splice(0)) {
      read.throw(error)
    }
  }
}

Usage.js

async function log(xs) {
  for await (let x of xs) {
    console.log(x)
  }
}

log(new On('mousemove', document.body))