我可以在 javascript 的任何地方使用异步函数吗？

Question

Can i use async funtion inside

XMLHttpRequest.addEventListener('readystatechange',async function(){

or

setTimeout(async function(){ 

or

element.addEventListener('click',async function(){

Is syntax like that correct/valid?

I need await function but when i use it inside non async functions like below i got error: "Uncaught SyntaxError: await is only valid in async functions"

Answer 1

All of the APIs in question expect functions which do not expect any return values. So the primary thing you gain is ability to use await inside the function. Which, per your comment, it seems like that's what you want. It only takes 3 minutes to test it yourself and see -

 setTimeout(async _ => { const res = await fetch(`https://httpbin.org/json`) const data = await res.json() console.log(data) }, 1000) console.log("loading...")

loading...

{
  "slideshow": {
    "author": "Yours Truly",
    "date": "date of publication",
    "slides": [
      {
        "title": "Wake up to WonderWidgets!",
        "type": "all"
      },
      {
        "items": [
          "Why <em>WonderWidgets</em> are great",
          "Who <em>buys</em> WonderWidgets"
        ],
        "title": "Overview",
        "type": "all"
      }
    ],
    "title": "Sample Slide Show"
  }
}

But you're missing the benefit of async and await by breaking out of the promise chain by using setTimeout in this way. It would be much better to write a promise around setTimeout instead. Rather than punting to the event queue, this keeps data moving through the promises and allows you to sequence any asynchronous portions of your program -

 const sleep = ms => new Promise(r => setTimeout(r, ms)) const fetchJson = (url, opts = {}) => fetch(url, opts).then(r => r.json()) async function main() { console.log("loading...") await sleep(1000) const data = await fetchJson(`https://httpbin.org/json`) console.log(data) return "done" } main().then(console.log, console.error)

loading...

{
  "slideshow": {
    "author": "Yours Truly",
    "date": "date of publication",
    "slides": [
      {
        "title": "Wake up to WonderWidgets!",
        "type": "all"
      },
      {
        "items": [
          "Why <em>WonderWidgets</em> are great",
          "Who <em>buys</em> WonderWidgets"
        ],
        "title": "Overview",
        "type": "all"
      }
    ],
    "title": "Sample Slide Show"
  }
}

done

The same applies for your addEventListner examples