diff --git a/src/WhatFsBackend.js b/src/WhatFsBackend.js
new file mode 100644
index 0000000..9ada414
--- /dev/null
+++ b/src/WhatFsBackend.js
@@ -0,0 +1,395 @@
+const { split } = require("path")
+
+class BadModeError extends Error {
+  static assert(flags, mode) {
+    if (mode === "r") {
+      if (!flags.read) {
+        throw new BadModeError(mode)
+      }
+      return
+    }
+    if (mode === "w") {
+      if (!flags.write) {
+        throw new BadModeError(mode)
+      }
+      return
+    }
+    if (mode === "a") {
+      if (!(flags.write && flags.append)) {
+        throw new BadModeError(mode)
+      }
+      return
+    }
+    throw new Error(`Asserting unknown mode '${mode}'`)
+  }
+}
+
+class InvalidFlagsError extends Error {
+  constructor(flags) {
+    super('Invalid Flags')
+    this.code = "ERR_INVALID_ARG_VALUE"
+    this.received = flags
+  }
+}
+
+class Flags {
+  static fallback(o, fallback) {
+    if (o) {
+      return new Flags(o)
+    } else {
+      return fallback
+    }
+  }
+
+  constructor(flags) {
+    this.reset(flags)
+  }
+
+  reset(flags, fallback) {
+    if (typeof(flags) === "string") {
+      this.resetString(flags, fallback)
+    } else {
+      this.resetObj(flags, fallback)
+    }
+    return this
+  }
+  resetObj(obj, fallback) {
+    if (obj?.read || obj?.write || obj?.append) {
+      this.append = obj.append
+      this.create = obj.create
+      this.mustCreate = obj.mustCreate
+      this.read = obj.read
+      this.sync = obj.sync
+      this.write = obj.write
+    } else if (fallback) {
+      this.reset(fallback)
+    } else {
+      throw new InvalidFlagsError(obj)
+    }
+    return this
+  }
+  resetString(str, fallback) {
+    const tooLong = str?.length > 3
+    if (!str?.length || tooLong) {
+      if (fallback && !tooLong) {
+        return this.reset(fallback)
+      } else {
+        throw new InvalidFlagsError(str)
+      }
+    }
+
+    const mode = str[0]
+    const append = mode === "a"
+    const read = mode === "r"
+    const write = mode === "w"
+
+    if (!(append || read || write)) {
+      throw new InvalidFlagsError(str)
+    }
+
+    let create = false
+    let mustCreate = false
+    let sync = false
+    for (let i = 1; i < str.length; ++i) {
+      const mod = str[i]
+      if (mod === "x") {
+        create = true
+        mustCreate = true
+      } else if (mod === "+") {
+        read = true
+        write = true
+        create = true
+      } else if (mod === "s") {
+        sync = true
+      } else {
+        throw new InvalidFlagsError(str)
+      }
+    }
+
+    this.append = append 
+    this.create = create
+    this.mustCreate = mustCreate
+    this.read = read
+    this.sync = sync
+    this.write = write
+    return this
+  }
+
+  toString() {
+    const str = `${this.read ? 'r' : ''}${this.append ? 'a' : this.write ? 'w' : ''}${this.mustCreate ? `x` : ''}${this.sync ? 's' : ''}`
+  }
+}
+
+class Noent extends Error {
+  constructor(path) {
+    super(`NOENT: no such file or directory, open '${path}'`)
+    this.errno = -2
+    this.code = "ENOENT"
+    this.syscall = "open"
+    this.path = path
+  }
+}
+
+const ReadFlags = new Flags('r')
+Object.freeze(ReadFlags)
+const WriteFlags = new Flags('w')
+Object.freeze(WriteFlags)
+
+module.exports = class WhatFsBackend {
+  static BadModeError = BadModeError
+  static InvalidFlagsError = InvalidFlagsError
+  static Flags = Flags
+
+  async _resolveDir(paths) {
+    let cursor = this._root
+    for (const i = 1; i < paths.length - 1; ++i) {
+      cursor = await cursor.getDirectoryHandle(paths[i])
+    }
+    return cursor
+  }
+  init() {
+  }
+  constructor(handle) {
+    this._root = handle
+  }
+  activate() {
+  }
+  deactivate() {
+  }
+  saveSuperblock() {
+  }
+  loadSuperblock() {
+  }
+  async readFile(filepath, opts) {
+    const flags = Flags.fallback(opts?.flags || ReadFlags)
+    BadModeError.assert(flags, 'r')
+    const { create } = flags
+    const paths = split(filepath)
+    const filename = paths[paths.length - 1]
+
+    try {
+      const dir = await this._resolveDir(paths)
+      const handle = await dir.getFileHandle(filename, { create })
+
+      // classic File API
+      //const file = await handle.getFile()
+      //return opts?.encoding === "utf8" ? file.text() : file.arrayBuffer()
+
+      const access = handle.createSyncAccessHandle()
+      const size = await handle.getSize()
+      const buffer = new ArrayBuffer(size)
+      access.read(buffer)
+      access.close()
+      
+      if (opts?.encoding === "utf8") {
+        const td = new TextDecoder()
+        return td.decode(buffer)
+      }
+      return buffer
+    } catch(e) {
+      throw Noent(filepath)
+    }
+  }
+  async writeFile(filepath, data, opts) {
+    const flags = Flags.fallback(opts?.flags || WriteFlags)
+    BadModeError.assert(flags, 'w')
+    const { append, create, mustCreate } = flags
+    const paths = split(filepath)
+
+    let dir
+    try {
+      dir = await this._resolveDir(paths)
+    } catch(err) {
+      throw new Noent(filepath)
+    }
+
+    const filename = paths[paths.length - 1];
+    flags?.mustCreate && await this._mustCreate(dir, filename);
+
+    try {
+      const handle = await dir.getFileHandle(filename, { create })
+ 
+      // classic-ish FileSystemWritableFileStream
+      //const position = append ? (await handle.getFile()).size : undefined
+      //const writable = handle.createWritable({ keepExistingData: append })
+      //await writable.write({ data, position: })
+      //await writable.close()
+      //return
+
+      const access = handle.createSyncAccessHandle()
+      const at = append ? await access.getSize() : undefined
+      if (opts?.encoding === "utf8" || typeof(data) === "string") {
+        const te = new TextEncoder()
+        data = te.encode(data).buffer
+      }
+
+      await access.write(buffer, { at })
+      await access.close()
+    } catch(e) {
+      throw Noent(filepath)
+    }
+  }
+  async unlink(filepath) {
+    const paths = split(filepath)
+    const filename = paths[paths.length - 1]
+    try {
+      const dir = await this._resolveDir(paths)
+      // classic File API only, WANTED: https://github.com/whatwg/fs/pull/9
+      await dir.removeEntry(filename)
+    } catch(e) {
+      throw Noent(filepath)
+    }
+  }
+  async readdir(filepath, opts) {
+    const paths = split(filepath)
+    const filename = paths[paths.length - 1]
+    try {
+      const dir = await this._resolveDir(paths)
+      return dir.keys()
+    } catch(e) {
+      throw Noent(filepath)
+    }
+  }
+  async mkdir(filepath, opts) {
+    const paths = split(filepath)
+    const last = paths.length - 1
+    const dirname = paths[last]
+    const recursive = opts?.recursive || false
+    let firstCreated
+    let cursor = this._root
+
+    for (const i = 1; i <= last; ++i) {
+      const path = paths[i]
+      let existing
+      try {
+        existing = await cursor.getDirectoryHandle(path)
+      } catch(err) {
+      }
+
+      if (existing) {
+        if (i === last && !recursive) {
+          throw new Error(`Directory '${filepath}' already existed`)
+        }
+        cursor = existing
+      } else if (recursive) {
+        if (!firstCreated) {
+          firstCreated = paths.slice(0, i).join("/")
+        }
+        cursor = await cursor.getDirectoryHandle(path, { create: true })
+      } else if (i === last) {
+        cursor = await cursor.getDirectoryHandle(path, { create: true })
+      } else {
+        throw new Noent(filepath)
+      }
+    }
+    return recursive ? firstCreated : undefined
+  }
+  async rmdir(filepath, opts) {
+    if (opts?.recursive) {
+      throw new Error("Deprecated 'recursive' rmdir not impmlemented")
+    }
+    return this.unlink(filepath)
+  }
+  async rename(oldFilepath, newFilepath) {
+    // WANTED: https://github.com/whatwg/fs/pull/10
+    const content = await this.readFile(oldFilepath)
+    await this.writeFile(newFilePath, content)
+    // for safety sake putting this last, at cost of extra disk usage
+    await this.unlink(oldFilePath)
+  }
+  async stat(filepath, opts) {
+    if (opts?.bigint) {
+      throw new Error("Stat 'bigint' option not implemented")
+    }
+    const paths = split(filepath)
+    const filename = paths[paths.length - 1]
+    try {
+      const dir = await this._resolveDir(paths)
+      const handle = await dir.getFileHandle(filename)
+      const file = await handle.getFile()
+      const mtimeMs = file.lastModified
+      // NEEDED: more metadata, https://github.com/whatwg/fs/issues/12
+      return {
+        size: file.size,
+        mtimeMs,
+        mtime: new Date(mtimeMs)
+      }
+    } catch(err) {
+      throw new Noent(filepath)
+    }
+  }
+  async lstat(filepath, opts) {
+    return this.stat(filepath)
+  }
+  async readlink(filepath, opts) {
+    // NEEDED: https://github.com/whatwg/fs/issues/54
+    throw new Error("Insufficient web standards for readlink");
+  }
+  async symlink(filepath, opts) {
+    // NEEDED: https://github.com/whatwg/fs/issues/54
+    throw new Error("Insufficient web standards for symlink");
+  }
+  async flush() {
+    // flush and cache would make sense if we kept a filepath->handle cache,
+    // which could definitely have other performance benefits
+  }
+  async close() {
+    // see `flush()` for some possibilities
+  }
+  async wipe() {
+    await this.rmdir("/", { recursive: true })
+  }
+  async watch() {
+    // NEEDED: https://github.com/WICG/file-system-access/issues/72
+    throw new Error("Insufficient web standards for watch")
+  }
+  async truncate(filepath, len = 0) {
+    const paths = split(filepath)
+    try {
+      const dir = await this._resolveDir(paths)
+      const handle = await dir.getFileHandle(paths[paths.length - 1])
+
+      // classic
+      //const writable = await handle.createWritable()
+      //await writable.truncate(len)
+      //await writable.close();
+
+      const access = await handle.createSyncAccessHandle()
+      await access.truncate(len)
+      await access.close()
+    } catch(err) {
+      throw new Noent(filepath)
+    }
+  }
+  async _mustCreate(dir, filename) {
+    let resultNotFound = false
+    try {
+      await dir.getFileHandle(filename)
+    } catch (err) {
+      // TODO: maybe check this harder
+      resultNotFound = true
+    }
+    if (!resultNotFound) {
+       throw new Error("File '${filepath}' already existed")
+    }
+  }
+  async copy(src, dest) {
+    const content = await this.readFile(src)
+    await this.writeFile(dest, content)
+  }
+  async access(filepath) {
+    // WANTED: more metadata, https://github.com/whatwg/fs/issues/12
+    // TODO: we could implement some horrible test of reading/writing
+    // but for now assume we have access if it exists
+    await this.stat(filepath)
+    // https://github.com/nodejs/node/blob/main/typings/internalBinding/constants.d.ts#L179
+    return 255;
+  }
+  async appendFile(path, data, opts) {
+    const mode = Flags.fallback(opts?.flags, 'a')
+    if (mode.write && !mode.append) {
+      throw new BadModeError(mode)
+    }
+    await this.writeFile(path, data, { ...opts, mode })
+  }
+}