如何将异步函数值传递给事件处理程序

Question

I'm fetching the stylesheet and replacing all CSS variables with the actual hex value it corresponds to when the user changes the color as desired.

I created an event handler so that when the user clicks the download button, all of the colors he/she selected, would be saved in the stylesheet at that moment, but it doesn't seem to work. I know it's an issue with my understanding of promises as a whole and async await

What I did.

const fetchStyleSheet = async () => {
  const res = await fetch("./themes/prism.css");
  const orig_css = await res.text();
  let updated_css = orig_css;

  const regexp = /(?:var\(--)[a-zA-z\-]*(?:\))/g;
  let cssVars = orig_css.matchAll(regexp);
  cssVars = Array.from(cssVars).flat();
  console.log(cssVars)

  for await (const variable of cssVars) {
    const trimmedVar = variable.slice(6, -1)
    const styles = getComputedStyle(document.documentElement)
    const value = String(styles.getPropertyValue(`--${trimmedVar}`)).trim()

    updated_css = updated_css.replace(variable, value);
  }
  console.log(updated_css)

  return updated_css
}

const main = async () => {
  const downloadBtn = document.getElementById('download-btn')
  downloadBtn.addEventListener('click', () => {
    const updated_css = fetchStyleSheet()
    downloadBtn.setAttribute('href', 'data:application/octet-stream;charset=utf-8,' + encodeURIComponent(updated_css))
    downloadBtn.setAttribute('download', 'prism-theme.css')
  })
}

main()

I can't await the updated_css because it falls into the callback of the click event, which is a new function.

Then I did the following thinking it would work since it was top level.

const downloadBtn = document.getElementById('download-btn')
downloadBtn.addEventListener('click', async () => {
  const updated_css = await fetchStyleSheet()
  downloadBtn.setAttribute('href', 'data:application/octet-stream;charset=utf-8,' + encodeURIComponent(updated_css))
  downloadBtn.setAttribute('download', 'prism-theme.css')
})

That gave me the following error TypeError: NetworkError when attempting to fetch resource.

I understand that calling fetchStyleSheet() only returns a promise object at first and to get the value (which is updated_css ), I need to follow it with .then() or await it.

Answer 1

The await is the correct approach to deal with the fetchStyleSheet() call returning a promise, your problem is that the click on the link tries to follow the href attribute immediately - before you set it to that data url. What you would need to do instead is prevent the default action, asynchronously do your stuff, and then re-trigger the click when you're done. Also don't forget to deal with possible exceptions:

const downloadBtn = document.getElementById('download-btn')
downloadBtn.addEventListener('click', async (event) => {
  if (!e.isTrusted) return // do nothing on the second run
  try {
    event.preventDefault()
    const updated_css = await fetchStyleSheet()
    downloadBtn.setAttribute('href', 'data:application/octet-stream;charset=utf-8,' + encodeURIComponent(updated_css))
    downloadBtn.setAttribute('download', 'prism-theme.css')
    downloadBtn.click() // simulate a new click
  } catch(err) {
    console.error(err) // or alert it, or put the message on the page
  }
})