Jaspr JS Interop

Rules for JS Interop

Dart makes accessing JavaScript simple using modern js_interop. Since Jaspr applications can be compiled to JavaScript or WebAssembly, utilizing Dart's explicit js_interop mechanism is required when you need to interface with raw JS variables, functions, or libraries.

1. Import Strategy

• Rule 1: In server or static mode, you MUST ALWAYS import package:universal_web/js_interop.dart. It acts as a safe stub during server-side processing without breaking compilation.
• Rule 2: In server or static mode, you MUST wrap any JS Interop execution in an if (kIsWeb) { ... } block to avoid throwing exceptions on the server during pre-rendering.
• Rule 3: In client mode, you can directly import dart:js_interop.
• Rule 4: For standard browser APIs (like window.innerHeight, document, DOM nodes), use package:universal_web/web.dart (or package:web/web.dart in client mode) instead of manually creating js_interop wrappers.

Usage Examples

Top Level Functions

To bind directly to a globally available JS function or object, use @JS():

@JS()
external void alert(JSString message);

void showAlert() {
  if (kIsWeb) {
    // Make sure to convert Dart String types using .toJS
    alert('Hello from Dart!'.toJS);
  }
}

Wrapping Third-Party JS Libraries

To interact with specific Javascript Objects from external libraries, map an extension type wrapping JSObject.

// Suppose there is a JS library available via `window.Analytics`
extension type Analytics._(JSObject _) implements JSObject {
  external void trackPage(JSString pageName);
  external JSNumber getVisitCount();
}

@JS('Analytics')
external Analytics get analytics;

void logCurrentPage() {
  if (kIsWeb) {
    analytics.trackPage('Home'.toJS);
    
    // Convert Javascript primitives back to Dart types using `.toDart`
    int visits = analytics.getVisitCount().toDartInt;
    print('Total visits: $visits');
  }
}

Anonymous JS Objects

If you need to pass a configuration object (a plain JS object literal {}) into a JS function, or receive one from JavaScript, define an @anonymous extension type.

@JS()
@anonymous
extension type ChartConfig._(JSObject _) implements JSObject {
  external factory ChartConfig({
    JSString title,
    JSBoolean showLegend,
    JSArray<JSNumber> data,
  });

  external JSString get title;
  external JSArray<JSNumber> get data;
}

@JS()
external void renderChart(ChartConfig config);

void setupChart() {
  if (kIsWeb) {
    // Creating an anonymous JS object to pass into a JS function
    final config = ChartConfig(
      title: 'Monthly Sales'.toJS,
      showLegend: true.toJS,
      data: [10.toJS, 20.toJS, 30.toJS].toJS,
    );
    
    renderChart(config);

    // Reading properties from a JS object back to Dart types
    String title = config.title.toDart;
    List<double> rawData = config.data.toDart.map((e) => e.toDartDouble).toList();
  }
}