1// Core picker methods2 3// Open file(s) for reading4const [handle] = await window.showOpenFilePicker({5  types: [6    {7      description: 'Text files',8      accept: { 'text/plain': ['.txt', '.md', '.json'] }9    }10  ],11  multiple: false12});13 14// Save file to disk15const saveHandle = await window.showSaveFilePicker({16  suggestedName: 'document.txt',17  types: [18    {19      description: 'Text document',20      accept: { 'text/plain': ['.txt'] }21    }22  ]23});24 25// Select directory for batch operations26const dirHandle = await window.showDirectoryPicker();27 28// Read file contents from handle29const file = await handle.getFile();30const contents = await file.text();31 32// Write to file via writable stream33const writable = await saveHandle.createWritable();34await writable.write('Hello, File System!');35await writable.close();

1// Reading files with getFile()2 3async function readTextFile(handle) {4  const file = await handle.getFile();5  return await file.text();6}7 8async function readJsonFile(handle) {9  const file = await handle.getFile();10  const text = await file.text();11  return JSON.parse(text);12}13 14async function readLargeFile(handle) {15  const file = await handle.getFile();16  const stream = file.stream();17  const reader = stream.getReader();18 19  while (true) {20    const { done, value } = await reader.read();21    if (done) break;22    // Process chunk (value is Uint8Array)23    console.log('Received chunk:', value.length, 'bytes');24  }25}26 27// Handle errors gracefully28async function safeReadFile(handle) {29  try {30    const file = await handle.getFile();31    return await file.text();32  } catch (error) {33    if (error.name === 'NotFoundError') {34      throw new Error('File has been deleted or moved');35    }36    throw error;37  }38}

1// Writing files with createWritable()2 3async function writeTextFile(handle, content) {4  const writable = await handle.createWritable();5  await writable.write(content);6  await writable.close();7}8 9async function writeJsonFile(handle, data) {10  const writable = await handle.createWritable();11  await writable.write(JSON.stringify(data, null, 2));12  await writable.close();13}14 15async function appendToFile(handle, content) {16  const writable = await handle.createWritable({ keepExistingData: true });17  await writable.seek(0); // Move to end of existing data18  await writable.write(content);19  await writable.close();20}21 22// Write from a blob or buffer23async function writeBuffer(handle, buffer) {24  const writable = await handle.createWritable();25  await writable.write(buffer);26  await writable.close();27}28 29// Using write options for truncate or append30async function writeWithOptions(handle, content, mode = 'truncate') {31  const writable = await handle.createWritable({32    keepExistingData: mode === 'append'33  });34 35  if (mode === 'truncate') {36    await writable.write({ size: 0 }); // Clear file37  }38 39  await writable.write(content);40  await writable.close();41}

1// Directory operations2 3// List all items in a directory4async function listDirectory(dirHandle) {5  const items = [];6  for await (const [name, handle] of dirHandle.entries()) {7    items.push({8      name,9      kind: handle.kind, // 'file' or 'directory'10      handle11    });12  }13  return items;14}15 16// Get or create nested handles17async function getOrCreateFile(dirHandle, filename) {18  return dirHandle.getFileHandle(filename, { create: true });19}20 21async function getOrCreateDir(dirHandle, dirname) {22  return dirHandle.getDirectoryHandle(dirname, { create: true });23}24 25// Remove a file or directory26async function removeItem(dirHandle, name) {27  await dirHandle.removeEntry(name, { recursive: true });28}29 30// Recursively walk directory tree31async function* walkDirectory(dirHandle, path = '') {32  for await (const [name, handle] of dirHandle.entries()) {33    const fullPath = path ? `${path}/${name}` : name;34    if (handle.kind === 'directory') {35      yield { path: fullPath, kind: 'directory', handle };36      yield* walkDirectory(handle, fullPath);37    } else {38      yield { path: fullPath, kind: 'file', handle };39    }40  }41}42 43// Count files in directory tree44async function countFiles(dirHandle) {45  let count = 0;46  for await (const item of walkDirectory(dirHandle)) {47    if (item.kind === 'file') count++;48  }49  return count;50}

1// Permission management2 3async function requestFilePermission(handle, readOnly = true) {4  const options = { mode: readOnly ? 'read' : 'readwrite' };5 6  // Check current permission state7  const status = await handle.queryPermission(options);8 9  if (status === 'granted') {10    return true;11  }12 13  if (status === 'denied') {14    return false;15  }16 17  // Request permission from user18  try {19    const result = await handle.requestPermission(options);20    return result === 'granted';21  } catch (error) {22    console.error('Permission request failed:', error);23    return false;24  }25}26 27// Wrapper for file operations with auto-permission28async function withFilePermission(handle, operation, readOnly = true) {29  const hasPermission = await requestFilePermission(handle, readOnly);30  if (!hasPermission) {31    throw new Error('File permission denied');32  }33  return operation(handle);34}35 36// Check permission without prompting37async function hasAccess(handle, mode = 'read') {38  try {39    const status = await handle.queryPermission({ mode });40    return status === 'granted';41  } catch {42    return false;43  }44}45 46// Persist handle and request persistent access47async function openWithPersistence() {48  const [handle] = await window.showOpenFilePicker();49 50  // Request persistent permission51  const opts = { mode: 'readwrite', keepPermission: true };52  if ((await handle.queryPermission(opts)) === 'prompt') {53    await handle.requestPermission(opts);54  }55 56  return handle;57}

1// Browser compatibility helpers2 3// Check if File System Access API is supported4function isFileSystemAccessSupported() {5  return 'showOpenFilePicker' in window;6}7 8// Feature detection with graceful fallback9async function openFileWithFallback() {10  if (isFileSystemAccessSupported()) {11    const [handle] = await window.showOpenFilePicker();12    const file = await handle.getFile();13    return { handle, file, method: 'fs-access' };14  }15 16  // Fallback to traditional file input17  return new Promise((resolve) => {18    const input = document.createElement('input');19    input.type = 'file';20    input.accept = '.txt,.md,.json';21 22    input.onchange = (event) => {23      const file = event.target.files[0];24      resolve({ file, method: 'file-input' });25    };26 27    input.click();28  });29}30 31// Detect specific capabilities32async function getBrowserCapabilities() {33  const caps = {34    picker: 'showOpenFilePicker' in window,35    directory: 'showDirectoryPicker' in window,36    writable: false,37    syncHandle: false38  };39 40  if (caps.picker) {41    try {42      const testHandle = await window.showSaveFilePicker();43      caps.writable = true;44      testHandle.handle.cancel();45    } catch (e) {46      // Feature exists but failed to use47    }48  }49 50  return caps;51}52 53// UI message based on support54function getUnsupportedMessage() {55  const ua = navigator.userAgent;56 57  if (ua.includes('Firefox')) {58    return 'Firefox: Enable dom.webshell.enabled and dom.fileHandle.enabled in about:config';59  }60  if (ua.includes('Safari') && !ua.includes('Chrome')) {61    return 'Safari: Enable Experimental Features > File System Access API in Develop menu';62  }63  return 'Your browser does not support the File System Access API. Try Chrome, Edge, or Opera.';64}

1// Complete Document Editor using File System Access API2 3class DocumentEditor {4  constructor() {5    this.handle = null;6    this.filename = 'Untitled';7    this.content = '';8    this.unsavedChanges = false;9  }10 11  // Check for API support12  static isSupported() {13    return 'showOpenFilePicker' in window;14  }15 16  // Open existing file17  async open() {18    try {19      const [handle] = await window.showOpenFilePicker({20        types: [21          {22            description: 'Text Documents',23            accept: {24              'text/plain': ['.txt', '.md', '.json', '.html', '.css', '.js']25            }26          }27        ]28      });29 30      return await this.loadFromHandle(handle);31    } catch (error) {32      if (error.name !== 'AbortError') {33        console.error('Failed to open file:', error);34      }35      return false;36    }37  }38 39  // Load content from file handle40  async loadFromHandle(handle) {41    try {42      // Request read permission43      if ((await handle.queryPermission({ mode: 'read' })) === 'prompt') {44        await handle.requestPermission({ mode: 'read' });45      }46 47      const file = await handle.getFile();48      this.content = await file.text();49      this.filename = file.name;50      this.handle = handle;51      this.unsavedChanges = false;52 53      return true;54    } catch (error) {55      console.error('Failed to load file:', error);56      return false;57    }58  }59 60  // Create new file (shows save dialog)61  async create() {62    try {63      const handle = await window.showSaveFilePicker({64        suggestedName: `${this.filename}.txt`,65        types: [66          {67            description: 'Text Document',68            accept: { 'text/plain': ['.txt'] }69          },70          {71            description: 'Markdown',72            accept: { 'text/markdown': ['.md'] }73          }74        ]75      });76 77      this.content = '';78      this.filename = handle.name;79      this.handle = handle;80      this.unsavedChanges = false;81 82      return true;83    } catch (error) {84      if (error.name !== 'AbortError') {85        console.error('Failed to create file:', error);86      }87      return false;88    }89  }90 91  // Save current file92  async save() {93    if (!this.handle) {94      return await this.saveAs();95    }96 97    try {98      // Request write permission if needed99      if ((await this.handle.queryPermission({ mode: 'readwrite' })) === 'prompt') {100        await this.handle.requestPermission({ mode: 'readwrite' });101      }102 103      const writable = await this.handle.createWritable();104      await writable.write(this.content);105      await writable.close();106 107      this.unsavedChanges = false;108      return true;109    } catch (error) {110      console.error('Failed to save:', error);111      return false;112    }113  }114 115  // Save as new file116  async saveAs() {117    try {118      const handle = await window.showSaveFilePicker({119        suggestedName: this.filename,120        types: [121          {122            description: 'Text Document',123            accept: { 'text/plain': ['.txt'] }124          }125        ]126      });127 128      this.handle = handle;129      this.filename = handle.name;130 131      return await this.save();132    } catch (error) {133      if (error.name !== 'AbortError') {134        console.error('Failed to save as:', error);135      }136      return false;137    }138  }139 140  // Update content141  updateContent(newContent) {142    this.content = newContent;143    this.unsavedChanges = true;144  }145 146  // Get current content147  getContent() {148    return this.content;149  }150 151  // Check for unsaved changes152  hasUnsavedChanges() {153    return this.unsavedChanges;154  }155 156  // Get filename157  getFilename() {158    return this.filename;159  }160}161 162// Usage example163const editor = new DocumentEditor();164 165// Hook up to UI166document.getElementById('open-btn').onclick = () => editor.open();167document.getElementById('save-btn').onclick = () => editor.save();168document.getElementById('new-btn').onclick = () => editor.create();169 170// Handle content changes171document.getElementById('editor').oninput = (e) => {172  editor.updateContent(e.target.value);173};

1// Origin Private File System (OPFS) for performance2 3class OPFSManager {4  constructor() {5    this.root = null;6  }7 8  // Initialize OPFS access9  async init() {10    this.root = await navigator.storage.getDirectory();11    return this;12  }13 14  // Get or create a file in OPFS15  async getFileHandle(name, options = {}) {16    if (!this.root) await this.init();17    return this.root.getFileHandle(name, options);18  }19 20  // Get or create a directory in OPFS21  async getDirectoryHandle(name, options = {}) {22    if (!this.root) await this.init();23    return this.root.getDirectoryHandle(name, options);24  }25 26  // Write data to OPFS file27  async writeToOPFS(filename, content) {28    const handle = await this.getFileHandle(filename, { create: true });29    const writable = await handle.createWritable();30    await writable.write(content);31    await writable.close();32  }33 34  // Read data from OPFS file35  async readFromOPFS(filename) {36    const handle = await this.getFileHandle(filename);37    const file = await handle.getFile();38    return await file.text();39  }40 41  // List all files in OPFS root42  async listFiles() {43    if (!this.root) await this.init();44    const files = [];45    for await (const [name, handle] of this.root.entries()) {46      files.push({ name, kind: handle.kind });47    }48    return files;49  }50 51  // Delete file from OPFS52  async deleteFromOPFS(filename) {53    if (!this.root) await this.init();54    await this.root.removeEntry(filename);55  }56}57 58// Performance optimization example59async function processLargeFileOptimized(userFileHandle) {60  const opfs = new OPFSManager();61  await opfs.init();62 63  // Read user file64  const userFile = await userFileHandle.getFile();65  const chunks = [];66 67  // Process in chunks using OPFS for intermediate storage68  const stream = userFile.stream();69  const reader = stream.getReader();70  let chunkIndex = 0;71 72  while (true) {73    const { done, value } = await reader.read();74    if (done) break;75 76    // Store chunk in OPFS77    await opfs.writeToOPFS(`chunk-${chunkIndex}.tmp`, value);78    chunks.push(`chunk-${chunkIndex}.tmp`);79    chunkIndex++;80  }81 82  // Process each chunk from OPFS83  const processedChunks = [];84  for (const chunkName of chunks) {85    const data = await opfs.readFromOPFS(chunkName);86    const processed = await transformData(data); // Your processing function87    processedChunks.push(processed);88 89    // Clean up temp file90    await opfs.deleteFromOPFS(chunkName);91  }92 93  // Combine results94  return processedChunks.join('');95}

1// Best practices for File System Access API2 3class RobustFileHandler {4  constructor() {5    this.currentHandle = null;6  }7 8  // 1. Always use try-catch with specific error handling9  async safeOpen() {10    try {11      const [handle] = await window.showOpenFilePicker();12      return handle;13    } catch (error) {14      switch (error.name) {15        case 'AbortError':16          console.log('User cancelled the file picker');17          return null;18        case 'SecurityError':19          throw new Error('File access blocked by security policy');20        default:21          console.error('Unknown error:', error);22          throw error;23      }24    }25  }26 27  // 2. Request permissions before critical operations28  async ensurePermission(handle, readOnly = true) {29    const options = { mode: readOnly ? 'read' : 'readwrite' };30 31    try {32      const status = await handle.queryPermission(options);33      if (status === 'granted') return true;34      if (status === 'denied') return false;35 36      const result = await handle.requestPermission(options);37      return result === 'granted';38    } catch (error) {39      console.error('Permission error:', error);40      return false;41    }42  }43 44  // 3. Check file availability before operations45  async isFileAvailable(handle) {46    try {47      const file = await handle.getFile();48      // If we can get the file, it's available49      return true;50    } catch (error) {51      if (error.name === 'NotFoundError') return false;52      throw error; // Other errors should propagate53    }54  }55 56  // 4. Handle concurrent modifications57  async saveWithConflictCheck(handle, newContent) {58    // Get current file modification time59    const file = await handle.getFile();60    const lastModified = file.lastModified;61 62    // Check if file has been modified since we opened it63    if (this.lastKnownModified && lastModified !== this.lastKnownModified) {64      const userWantsMerge = confirm(65        'The file has been modified elsewhere. Overwrite anyway?'66      );67      if (!userWantsMerge) return false;68    }69 70    // Proceed with save71    const writable = await handle.createWritable();72    await writable.write(newContent);73    await writable.close();74 75    // Update our tracking76    this.lastKnownModified = (await handle.getFile()).lastModified;77    return true;78  }79 80  // 5. Provide undo capability with OPFS81  async createSnapshot(content, operation) {82    const opfs = await navigator.storage.getDirectory();83    const snapshotName = `snapshot-${Date.now()}-${operation}.txt`;84 85    const handle = await opfs.getFileHandle(snapshotName, { create: true });86    const writable = await handle.createWritable();87    await writable.write(content);88    await writable.close();89 90    return snapshotName;91  }92}93 94// 6. Use AbortController for cancellable operations95class CancellableFileReader {96  constructor() {97    this.abortController = new AbortController();98  }99 100  async readLargeFile(handle) {101    try {102      const file = await handle.getFile();103      const stream = file.stream();104      const reader = stream.getReader();105 106      while (true) {107        const { done, value } = await reader.read({108          signal: this.abortController.signal109        });110        if (done) break;111        // Process value...112      }113    } catch (error) {114      if (error.name === 'AbortError') {115        console.log('Read operation was cancelled');116        return;117      }118      throw error;119    }120  }121 122  cancel() {123    this.abortController.abort();124  }125}