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File Handling Access API


In The File System Access API: simplifying access to local files, the authors describe how to use the API to create file management functionality for web applications

The idea is that we'll be able to open a file from, display it in the browser and then if desired, save it to a new location in our local file system.

To open a file we'll use the following code.

We define the following constants and variables:

  • Capture the button to open a file
  • Capture the button to save a file
  • Capture the text area where we'll insert the text
  • Capture the content that we'll save
  • Sets up an empty variable for the filehandle we want to work with
const openButton = document.querySelector('.openButton');
const saveButton = document.querySelector('.saveButton');
const display = document.querySelector('.content-area');
const dataToSave = display.innerText;
let fileHandle;

The openFile function is called when the open file button is clicked. It opens the file, reads the content, and puts the content in the text area.

The function runs the following steps:

  1. We use feature detection to make sure the feature is supported, if not then we bail
  2. We create a new filehandle and show the file picker dialogue
  3. Open the file
  4. Read and convert the content of the file to text
  5. Insert the content of the file into the display text area
async function openFile() {
  if ('showOpenFilePicker' in window) { // 1
    [fileHandle] = await window.showOpenFilePicker(); // 2
    const file = await fileHandle.getFile(); // 3
    const contents = await file.text(); // 4
    display.innerText = contents; // 5
  } else {
    console.error('filehandle API not supported');

Saving a file #

Saving a file is a little more complicated. We need to create a new filehandle, capture the content to save, write the content to the file and then close the file and save it.

The steps are as follows:

  1. Use a feature query to detect if showSaveFilePicker is supported. If it's not then bail
  2. Show the save file picker
  3. Create a FileSystemWritableFileStream to write to
  4. Write the file
  5. close the file and write the contents to disk.
async function saveFile() {
  if ('showSaveFilePicker' in window) { // 1
    const newHandle = await window.showSaveFilePicker(); // 2
      const writableStream = await newHandle.createWritable(); // 3
        await writableStream.write(dataToSave); // 4
      await writableStream.close(); // 5

Finally, we attach the functions to the click event of the corresponding button.

openButton.addEventListener('click', openFile);
saveButton.addEventListener('click', saveFile);

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