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README.md |
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()`.
}
@staticInterop
With package:js
, we have historically had two different types of classes:
plain @JS
(those with just the @JS
annotation) and @anonymous
classes.
Now, you can use a new one: @staticInterop
.
These classes are different in that they do not allow instance members within the class itself. All such members need to go into an extension (hence “static”). Let’s look at an example:
@JS()
library static_interop;
import 'package:js/js.dart';
// Assumes there is a top-level `StaticInterop` class in a JS module.
@JS()
@staticInterop
class StaticInterop {
external factory StaticInterop();
}
extension on StaticInterop {
external int field;
external int get getSet;
external set getSet(int val);
external int method();
}
void main() {
var jsObj = StaticInterop();
jsObj.field = 1;
jsObj.method();
}
The external
static extension members get lowered to JS naturally:
jsObj.field
becomes a property get of field
in JS and jsObj.method()
becomes a function invocation of method
on jsObj
.
In many ways, these classes are just like the plain @JS
and @anonymous
classes. Like with plain @JS
classes, you can provide a value in @JS
if you
want the constructor to use a particular JS class e.g.
@JS(‘module.MyJSClass’)
. You can also add @anonymous
to @staticInterop
classes if you want the factory constructor with named arguments in order to
make an object literal e.g.
external factory AnonymousStaticInterop({int? field1, int? field2})
. Also like
with plain @JS
classes, you can’t inherit non-package:js
classes. You should
only inherit other @staticInterop
classes for subtyping and inheriting
extension methods. Lastly, you can freely cast JS objects to and from the three
types of package:js
classes.
What makes @staticInterop
unique, however, is that you can use them to
represent DOM objects as well as other JS objects, which you can’t with previous
package:js
classes. Historically, you’ve needed to use dart:html
to interact
with the DOM e.g. DivElement
. Now, you can create your own abstraction for
these objects instead of using the ones we provide in dart:html
:
@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 you can have both external
and non-external
members in the
extension.
Compared to non-@staticInterop
package:js
classes, @staticInterop
classes:
- Are more performant
- Have better type guarantees
- Generate less code
- Allow non-
external
members - Allow
external
extension members to be renamed using@JS()
e.g.@JS('renamedField')
The only catch is that virtual/dynamic dispatch is disallowed. That means methods are resolved using only the static type of the object.
In general, it's advised to use @staticInterop
wherever you can, as future
JS interop will only target static dispatch.
@JSExport and js_util.createDartExport
One of the difficulties with JS interop is that most of it is exclusively
focused on importing JS code to Dart, not the other way around. We have some
functionality like allowInterop
, which allows you to call Dart functions in
JS, but this becomes cumbersome when you want to use a Dart object. You need to
essentially allowInterop
all members manually.
createDartExport
instead lets you do this automatically. Let’s see how with an
example:
import 'dart:js_util';
import 'package:expect/minitest.dart';
import 'package:js/js.dart';
// The Dart class must have `@JSExport` on it or one of its instance members.
@JSExport()
class Counter {
int value = 0;
@JSExport('increment')
void renamedIncrement() {
value++;
}
}
@JS()
@staticInterop
class JSCounter {}
extension on JSCounter {
external int value;
external void increment();
}
void main() {
var dartCounter = Counter();
var counter = createDartExport<Counter>(dartCounter) as JSCounter;
expect(counter.value, 0);
counter.increment();
expect(counter.value, 1);
expect(dartCounter.value, 1); // Dart object gets modified
dartCounter.value = 0;
expect(counter.value, 0); // Changes in Dart object affect the exported object
}
There are a number of things happening here. At a high level, you pass
createDartExport
an instance of some Dart object that has @JSExport
either
on it or one of its instance members and the object’s static type if needed.
Using the static type, we transform the createDartExport
call into a JS object
literal that is a mapping from each member’s Dart name (accounting for renames
using the @JSExport
annotation) to the member. The JS object essentially wraps
and acts as a proxy to the exported Dart object.
Now, when we use it as a JS object (in this case, using @staticInterop
), we
can use the same names to access these members. We can also use the same syntax
to access these members e.g. counter.value = 0
. This now gives us an easy to
do what we wanted before with allowInterop
for each member.
There are, of course, limitations.
The only members that are “exported” are concrete instance members i.e. fields, getters, setters, and methods. That means you can’t export static members, constructors, factories, operators (the syntax complicates things), and extension methods. You can still have these members - they just won’t be present in the resulting exported object. Of course, you can use another instance member to call these members as well, and that instance member will be exported.
In order to use createDartExport
, you need to have a class that uses
@JSExport
.If you want to export only some members of a class, omit the
annotation on the class, and only use it on the members you want. If you need to
rename members, you can provide the @JSExport
annotation on that member a
string value, similar to renaming done via @JS()
. Inheritance respects the
individual superclass’ annotations. In other words, if the class of the object
you want to export has a superclass, but that superclass has no @JSExport
annotation anywhere, none of its superclass’ members are exported.
Lastly, different members can’t have the same export name, unless they are a getter and setter pair. So, for example, if you have a field and a method and one of them is renamed to the other’s name, that’s a conflict:
@JSExport()
class DartClass {
int member = 0;
@JSExport('member') // Two incompatible members have the same export name.
void method() {}
}
This holds true with inheritance as well, unless the member is overridden.
js_util.createStaticInteropMock
One of the neat things about the above example with Counter
is we’ve
essentially created a mock for JSCounter
. In the past, to mock a plain @JS
or @anonymous
class, you could create a Dart class that implements
that
interop class, and due to Dart's virtual dispatch, this would call the Dart
class' members instead. Now that we're using external
extension members, this
no longer works. We now have to mock at the JS level instead. With
createDartExport
, you’re essentially using a Dart object to replace a JS
object. This functionality is equivalent to mocking at the JS level, and you can
also use it to mock the old non-@staticInterop
package:js
classes!
One useful feature of the old style of mocking using implements
is it lets you
know if you've implemented the needed members. We can't do that with
createDartExport
. For example:
@JSExport()
class Counter {
// Where is `value` and `increment`?
}
This would obviously not be a satisfactory mock for JSCounter
.
createDartExport
has no idea what class you're trying to mock, so it can't
tell you if you’ve got your mock class right.
This is where createStaticInteropMock
comes in. It takes in a separate type
argument, e.g. createStaticInteropMock<JSCounter, Counter>(Counter())
, to
determine whether mocking conformance is satisfied. This type argument must be
a @staticInterop
class. With this, you’ll see an error saying that you haven’t
implemented all the needed members. If the mock class implements all the needed
members, the function does the same thing as createDartExport
, and returns an
object literal that wraps the Dart object.
You can also use package:mockito
to do the mocking with this API, by providing
a generated mocking object from package:mockito
to createStaticInteropMock
.
There are some corner cases here that are worth noting.
It is possible, through the expressiveness of extension methods, to have name conflicts like this:
@JS()
@staticInterop
class StaticInterop {}
extension A on StaticInterop {
external Function member;
}
extension B on StaticInterop {
external void member();
}
This present an issue as a single Dart class cannot implement member
as both a
field and a function. So, what to do? We require that you only implement one
of these members. So, either a Function field or a function are satisfactory.
It is also sometimes desired that the mocking object is the same underlying type
as the JS object you are interfacing. For example, if you want to mock a JS
Element
, you’d want the type of the mocking object to also be a Element
in
order to pass instanceof
checks. In order to do this, we let users pass the JS
prototype of the type they want the mocking object to be as an argument to
createStaticInteropMock
.
An important note here is that createStaticInteropMock
looks for all
extensions of the @staticInterop
type in the program, even if they are out of
scope of the current file. In order to avoid a case where other libraries
extending the @staticInterop
type break your usage of
createStaticInteropMock
, you should try to only use this API in tests.
createStaticInteropMock
is meant to detect issues earlier at compile-time, but
if it's too restrictive, you can still use createDartExport
to workaround
that (and please provide us feedback on why it's restrictive!).
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. This is not true for @staticInterop
classes unless the
@trustTypes
annotation is used.
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.