explicity truncate files when not keeping existing data

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
     }