feat: 🎸 improve writing at offset logic

src/node-to-fsa/NodeFileSystemFileHandle.ts

@@ -51,8 +51,12 @@ export class NodeFileSystemFileHandle extends NodeFileSystemHandle {
    * @see https://developer.mozilla.org/en-US/docs/Web/API/FileSystemFileHandle/createWritable
    */
   public async createWritable(
-    { keepExistingData = false }: { keepExistingData?: boolean } = { keepExistingData: false },
+    { keepExistingData = false }: CreateWritableOptions = { keepExistingData: false },
   ): Promise<NodeFileSystemWritableFileStream> {
-    return new NodeFileSystemWritableFileStream(this.fs, this.__path);
+    return new NodeFileSystemWritableFileStream(this.fs, this.__path, keepExistingData);
   }
 }
+
+export interface CreateWritableOptions {
+  keepExistingData?: boolean;
+}

src/node-to-fsa/NodeFileSystemWritableFileStream.ts

@@ -1,6 +1,11 @@
 import type { IFileHandle } from '../promises';
 import type { NodeFsaFs } from './types';
 
+interface Ref {
+  handle: IFileHandle | undefined;
+  offset: number;
+}
+
 /**
  * Is a WritableStream object with additional convenience methods, which
  * operates on a single file on disk. The interface is accessed through the
@@ -9,21 +14,22 @@ import type { NodeFsaFs } from './types';
  * @see https://developer.mozilla.org/en-US/docs/Web/API/FileSystemWritableFileStream
  */
 export class NodeFileSystemWritableFileStream extends WritableStream {
-  protected handle: IFileHandle | undefined = undefined;
+  protected readonly ref: Ref;
 
-  constructor(protected readonly fs: NodeFsaFs, protected readonly path: string) {
-    const ref: { handle: IFileHandle | undefined } = { handle: undefined };
+  constructor(protected readonly fs: NodeFsaFs, protected readonly path: string, keepExistingData: boolean) {
+    const ref: Ref = { handle: undefined, offset: 0 };
     super({
       async start() {
-        ref.handle = await fs.promises.open(path, 'w');
+        ref.handle = await fs.promises.open(path, keepExistingData ? 'a+' : 'w');
       },
       async write(chunk: Data) {
         const handle = ref.handle;
         if (!handle) throw new Error('Invalid state');
         const buffer = Buffer.from(
           typeof chunk === 'string' ? chunk : chunk instanceof Blob ? await chunk.arrayBuffer() : chunk,
         );
-        await handle.write(buffer);
+        const { bytesWritten } = await handle.write(buffer, 0, buffer.length, ref.offset);
+        ref.offset += bytesWritten;
       },
       async close() {
         if (ref.handle) await ref.handle.close();
@@ -32,6 +38,7 @@ export class NodeFileSystemWritableFileStream extends WritableStream {
         if (ref.handle) await ref.handle.close();
       },
     });
+    this.ref = ref;
   }
 
   /**
@@ -40,7 +47,7 @@ export class NodeFileSystemWritableFileStream extends WritableStream {
    *                 (beginning) of the file.
    */
   public async seek(position: number): Promise<void> {
-    throw new Error('Not implemented');
+    this.ref.offset = position;
   }
 
   /**

src/node-to-fsa/__tests__/NodeFileSystemFileHandle.test.ts

@@ -27,7 +27,7 @@ maybe('NodeFileSystemFileHandle', () => {
 
   describe('.createWritable()', () => {
     describe('.write(chunk)', () => {
-      test('can write to file', async () => {
+      test('overwrites the file when write is being executed', async () => {
         const { dir, fs } = setup({
           'file.txt': 'Hello, world!',
         });
@@ -37,6 +37,19 @@ maybe('NodeFileSystemFileHandle', () => {
         await writable.close();
         expect(fs.readFileSync('/file.txt', 'utf8')).toBe('...');
       });
+
+      test('can seek and then write', async () => {
+        const { dir, fs } = setup({
+          'file.txt': '...',
+        });
+        const entry = await dir.getFileHandle('file.txt');
+        const writable = await entry.createWritable({keepExistingData: true});
+        writable.seek(1);
+        await writable.write('1');
+        await writable.write('2');
+        await writable.close();
+        expect(fs.readFileSync('/file.txt', 'utf8')).toBe('.12');
+      });
     });
   });
 });