dart-sdk/pkg/js

Use this package when you want to call JavaScript APIs from Dart code, or vice versa.

This package's main library, js, provides annotations and functions that let you specify how your Dart code interoperates with JavaScript code. The Dart-to-JavaScript compilers — dartdevc and dart2js — recognize these annotations, using them to connect your Dart code with JavaScript.

Important: This library supersedes dart:js, so don't import dart:js. Instead, import package:js/js.dart.

A second library in this package, js_util, provides low-level utilities that you can use when it isn't possible to wrap JavaScript with a static, annotated API.

Example

See the Chart.js Dart API for an end-to-end example.

Usage

The following examples show how to handle common interoperability tasks.

Calling JavaScript functions

@JS()
library stringify;

import 'package:js/js.dart';

// Calls invoke JavaScript `JSON.stringify(obj)`.
@JS('JSON.stringify')
external String stringify(Object obj);

Using JavaScript namespaces and classes

@JS('google.maps')
library maps;

import 'package:js/js.dart';

// Invokes the JavaScript getter `google.maps.map`.
external Map get map;

// The `Map` constructor invokes JavaScript `new google.maps.Map(location)`
@JS()
class Map {
  external Map(Location location);
  external Location getLocation();
}

// The `Location` constructor invokes JavaScript `new google.maps.LatLng(...)`
//
// We recommend against using custom JavaScript names whenever
// possible. It is easier for users if the JavaScript names and Dart names
// are consistent.
@JS('LatLng')
class Location {
  external Location(num lat, num lng);
}

Passing object literals to JavaScript

Many JavaScript APIs take an object literal as an argument. For example:

// JavaScript
printOptions({responsive: true});

If you want to use printOptions from Dart a Map<String, dynamic> would be "opaque" in JavaScript.

Instead, create a Dart class with both the @JS() and @anonymous annotations.

@JS()
library print_options;

import 'package:js/js.dart';

void main() {
  printOptions(Options(responsive: true));
}

@JS()
external printOptions(Options options);

@JS()
@anonymous
class Options {
  external bool get responsive;

  // Must have an unnamed factory constructor with named arguments.
  external factory Options({bool responsive});
}

Making a Dart function callable from JavaScript

If you pass a Dart function to a JavaScript API as an argument, wrap the Dart function using allowInterop() or allowInteropCaptureThis().

To make a Dart function callable from JavaScript by name, use a setter annotated with @JS().

@JS()
library callable_function;

import 'package:js/js.dart';

/// Allows assigning a function to be callable from `window.functionName()`
@JS('functionName')
external set _functionName(void Function() f);

/// Allows calling the assigned function from Dart as well.
@JS()
external void functionName();

void _someDartFunction() {
  print('Hello from Dart!');
}

void main() {
  _functionName = allowInterop(_someDartFunction);
  // JavaScript code may now call `functionName()` or `window.functionName()`.
}

Interop with native types using @staticInterop

Previously, you could not use @JS() or @anonymous types to interface with native types that were reserved within dart:html e.g. Window.

Using @staticInterop will now let you do so. However, it requires that there be no instance members within the class (constructors are still allowed). You can use static extension methods instead to declare these members. For example:

@JS()
library static_interop;

import 'dart:html' as html;

import 'package:js/js.dart';

@JS()
@staticInterop
class JSWindow {}

extension JSWindowExtension on JSWindow {
  external String get name;
  String get nameAllCaps => name.toUpperCase();
}

void main() {
  var jsWindow = html.window as JSWindow;
  print(jsWindow.name.toUpperCase() == jsWindow.nameAllCaps);
}

Note that in the above you can have both external and non-external members in the extension. You can have external variables, getters/setters, and methods within a static extension currently. These external members are lowered to their respective js_util calls under the hood. For example, the external name getter is equivalent to js_util.getProperty(this, 'name').

In general, it's advised to use @staticInterop wherever you can over using just @JS(). There will be fewer surprises and it's aligned with the statically typed future planned for JS interop.

Reporting issues

Please file bugs and feature requests on the SDK issue tracker.

Known limitations and bugs

Differences between dart2js and dartdevc

Dart's production and development JavaScript compilers use different calling conventions and type representation, and therefore have different challenges in JavaScript interop. There are currently some known differences in behavior and bugs in one or both compilers.

Dartdevc and dart2js have different representation for Maps

Passing a Map<String, String> as an argument to a JavaScript function will have different behavior depending on the compiler. Calling something like JSON.stringify() will give different results.

Workaround: Only pass object literals instead of Maps as arguments. For json specifically use jsonEncode in Dart rather than a JS alternative.

Missing validation for anonymous factory constructors in dartdevc

When using an @anonymous class to create JavaScript object literals dart2js will enforce that only named arguments are used, while dartdevc will allow positional arguments but may generate incorrect code.

Workaround: Try builds in both development and release mode to get the full scope of static validation.

Common problems

Dart and JavaScript have different semantics and common patterns, which makes it easy to make some mistakes and difficult for the tools to provide safety. These common problems are also known as sharp edges.

Lack of runtime type checking

The return types of methods annotated with @JS() are not validated at runtime, so an incorrect type may "leak" into other Dart code and violate type system guarantees.

Workaround: For any calls into JavaScript code that are not known to be safe in their return values, validate the results manually with is checks.

List instances coming from JavaScript will always be List<dynamic>

A JavaScript array does not have a reified element type, so an array returned from a JavaScript function cannot make guarantees about it's elements without inspecting each one. At runtime a check like result is List may succeed, while result is List<String> will always fail.

Workaround: Use .cast() or construct a new List to get an instance with the expected reified type. For instance if you want a List<String> use .cast<String>() or List<String>.from.

The JsObject type from dart:js can't be used with @JS() annotation

JsObject and related code in dart:js uses a different approach and may not be passed as an argument to a method annotated with @JS().

Workaround: Avoid importing dart:js and only use the package:js provided approach. To handle object literals use @anonymous on an @JS() annotated class.

is checks and as casts between JS interop types will always succeed

For any two @JS() types, with or without @anonymous, a check of whether an object of one type is another type will always return true, regardless of whether those two types are in the same prototype chain. Similarly, an explicit cast using as will also succeed.