const dir_ref = ... // Some mechanism to obtain a directory handle TBD.

for await (const [name, entry] of dir_ref) {
    // Iterate through a directory.
    // entry is a FileSystemFileHandle or a FileSystemDirectoryHandle.
    // name is equal to entry.name
}

const new_file = await dir_ref.getFile('foo.txt', {create: true}); // Create a file.

// Write to a file using an API resembling something like pwrite.
const new_file_writer = await new_file.createWritable();
await new_file_writer.write(await file_ref.getFile());
await new_file_writer.close();

// Or using streams.
const file_ref = await dir_ref.getFile('foo.js'); // Get a file from a directory.
const copy2 = await dir_ref.getFile('foo2.txt', {create: true});
(await file_ref.getFile()).stream().pipeTo(await copy2.createWritable());
...