fix: 🐛 correctly write at offset in FS adapter

src/crud/__tests__/matryoshka.test.ts

@@ -25,6 +25,35 @@ onlyOnNode20('CRUD matryoshka', () => {
   });
 
   describe('crud(fs(fsa(memfs)))', () => {
+    test('can write at offset', async () => {
+      const { fs } = memfs();
+      await fs.promises.writeFile('/file1.txt', 'abc');
+      const handle1 = await fs.promises.open('/file1.txt', 'r+');
+      await handle1.write(Buffer.from('.'), 0, 1, 1);
+      expect(await fs.promises.readFile('/file1.txt', 'utf8')).toBe('a.c');
+
+      const fsa = new NodeFileSystemDirectoryHandle(fs, '/', { mode: 'readwrite' });
+      const file2 = await fsa.getFileHandle('file2.txt', { create: true });
+      const writable1 = await file2.createWritable({ keepExistingData: false });
+      await writable1.write('abc');
+      await writable1.close();
+      const writable2 = await file2.createWritable({ keepExistingData: true });
+      await writable2.write({
+        type: 'write',
+        data: Buffer.from('.'),
+        position: 1,
+        size: 1,
+      });
+      await writable2.close();
+      expect(await (await file2.getFile()).text()).toBe('a.c');
+
+      const fs2 = new FsaNodeFs(fsa);
+      await fs2.promises.writeFile('/file3.txt', 'abc');
+      const handle = await fs2.promises.open('/file3.txt', 'r+');
+      await handle.write(Buffer.from('.'), 0, 1, 1);
+      expect(await fs2.promises.readFile('/file3.txt', 'utf8')).toBe('a.c');
+    });
+
     testCrudfs(() => {
       const { fs } = memfs();
       const fsa = new NodeFileSystemDirectoryHandle(fs, '/', { mode: 'readwrite' });

src/fsa-to-node/FsaNodeFsOpenFile.ts

@@ -31,7 +31,9 @@ export class FsaNodeFsOpenFile {
 
   public async write(data: ArrayBufferView, seek: number | null): Promise<void> {
     if (typeof seek !== 'number') seek = this.seek;
-    const writer = await this.file.createWritable({ keepExistingData: this.keepExistingData });
+    else this.keepExistingData = true;
+    const keepExistingData = this.keepExistingData;
+    const writer = await this.file.createWritable({ keepExistingData });
     await writer.write({
       type: 'write',
       data,

src/node/FileHandle.ts

@@ -215,7 +215,8 @@ export class FileHandle extends EventEmitter implements IFileHandle {
     length?: number,
     position?: number | null,
   ): Promise<TFileHandleWriteResult> {
-    const writePosition = position !== null && position !== undefined ? position : this.position;
+    const useInternalPosition = typeof position !== 'number';
+    const writePosition: number = useInternalPosition ? this.position : position;
 
     const result = await promisify(this.fs, 'write', bytesWritten => ({ bytesWritten, buffer }))(
       this.fd,
@@ -226,7 +227,7 @@ export class FileHandle extends EventEmitter implements IFileHandle {
     );
 
     // Update internal position only if position was null/undefined
-    if (position === null || position === undefined) {
+    if (useInternalPosition) {
       this.position += result.bytesWritten;
     }