How to read a file using showOpenFilePicker (NOT <input type="file">)

I'm not sure where that code came from, but the correct way to use that API looks like this. Assuming you have a file foo.txt with the contents "Hello, world!" in it:

// Returns a Promise of a single File object
async function pickSingleFile() {
  const [fileHandle] = await window.showOpenFilePicker();
  return await fileHandle.getFile();
}

// Takes a File object, returns a Promise of a string
async function readFile(file) {
  return await file.text();
}

pickSingleFile() // user picks foo.txt
  .then(readFile)
  .then(console.log) // prints "Hello, world!"
  .catch(console.error); // in case of oops

You can actually test this here, on this page, by creating a text file, pasting that code into your browser console (assuming you're on desktop Chrome or Edge) and navigating to the correct file in the file picker that opens.

Note as I said in the comments this feature has at the time of this writing has abysmal support and works only on desktop versions of Chromium-based browsers, no Safari, no FireFox, and no mobile (not even Chrome on Android), so think carefully before using.

What ended up working for me was:

fio.importFromFileOpenPicker = async function(fileHandle, fnImportToUI) {
  let file = await fileHandle.getFile();
  const reader = new FileReader();
  reader.addEventListener('loadend', (event) => {
    fnImportToUI(file, event.target.result)
  });
  reader.readAsText(file);
}

Thank you all for the help!

There are several mistakes in your snippet, I have fixed based on my assumptions. Code is pretty much self explanatory.

/**
 * convert a File object into base64 dataurl or plain text, returns null if its aborted
 * @param {File} file
 * @returns {Promise<string|null>}
 * @param {'plain'|'base64'} mode
 */
 
const convertFileToDataURLOrString = (file, mode) =>
  new Promise((resolve, reject) => {
    const reader = new FileReader()
    reader.addEventListener('load', () => resolve('' + reader.result))
    reader.addEventListener('abort', () => resolve(null))
    reader.addEventListener('error', () => reject(reader.error))
    if (mode === 'plain') reader.readAsText(file)
    else reader.readAsDataURL(file)
  })

fio.importFromFileButton = async function (fileHandle, fnImportToUI) {
  if (Array.isArray(fileHandle)) {
    fileHandle = fileHandle[0]
  }
  const datastr = convertFileToDataURLOrString(fileHandle, 'plain')
  // do something with datastr
}