explicity truncate files when not keeping existing data

scripts/test.ts

@@ -5,8 +5,11 @@ const cwd_directory_handle = new NodeFileSystemDirectoryHandle('.')
 const test_directory_handle = await cwd_directory_handle.getDirectoryHandle('test', { 'create': true })
 for (let i = 0; i < 10000; i++) {
     const file_handle = await test_directory_handle.getFileHandle(i+'.txt', { 'create': true })
-    const writable = await file_handle.createWritable()
+    let writable = await file_handle.createWritable()
-    await writable.write('Hello World!')
+    await writable.write(textText+textText)
+    await writable.close()
+    writable = await file_handle.createWritable()
+    await writable.write(textText)
     await writable.close()
     const file = await file_handle.getFile()
     const text = await file.text()

src/NodeFileSystemWriteableFileStream.ts

@@ -14,8 +14,9 @@ export default class NodeFileSystemWritableFileStream extends WritableStream imp
     constructor(name: string, options: FileSystemCreateWritableOptions = {}) {
         name = path.normalize(name)
         let flags = 'w'
-        if (!options.keepExistingData) flags = 'r+'
+        if (options.keepExistingData) flags = 'r+'
         const fd = fs.openSync(name, flags)
+        if (!options.keepExistingData) fs.ftruncateSync(fd, 0)
         super(Writable.toWeb(fs.createWriteStream(name, { "fd": fd })))
         this.#fd = fd
     }