Module testing 

Opt-in test fakes for browser APIs Playwright cannot drive natively.

The File System Access API (window.showSaveFilePicker / showOpenFilePicker) opens native OS dialogs with no DOM presence, so no locator or dialog handler can reach them. The standard cross-binding pattern is to install a deterministic fake before the app’s JS runs; FakeFileSystem packages that pattern so consumers don’t hand-roll it.

Nothing here is installed unless a test asks for it: a page that never calls Page::fake_file_system keeps the browser’s real (or absent) picker functions, so feature-detection and fallback paths stay testable.

Example

let fs = page.fake_file_system().await?;

// Seed a file the app's Open dialog will "pick":
fs.set_open_file("plan.json", br#"{"rooms": []}"#).await?;

// ... drive the app's Save As flow, then assert what it wrote:
page.goto("https://localhost:8080", None).await?;
let saved = fs.last_saved_bytes().await?;
assert!(saved.is_some());

Structs

FakeFileSystem

Handle to the fake File System Access API installed on a Page by Page::fake_file_system.