Support for interoperating with JavaScript.

This library provides access to JavaScript objects from Dart, allowing Dart code to get and set properties, and call methods of JavaScript objects and invoke JavaScript functions. The library takes care of converting between Dart and JavaScript objects where possible, or providing proxies if conversion isn't possible.

This library does not yet make Dart objects usable from JavaScript, their methods and proeprties are not accessible, though it does allow Dart functions to be passed into and called from JavaScript.

JsObject is the core type and represents a proxy of a JavaScript object. JsObject gives access to the underlying JavaScript objects properties and methods. JsObjects can be acquired by calls to JavaScript, or they can be created from proxies to JavaScript constructors.

The top-level getter context provides a JsObject that represents the global object in JavaScript, usually window.

The following example shows an alert dialog via a JavaScript call to the global function alert():

import 'dart:js';

main() => context.callMethod('alert', ['Hello from Dart!']);

This example shows how to create a JsObject from a JavaScript constructor and access its properties:

import 'dart:js';

main() {
  var object = new JsObject(context['Object']);
  object['greeting'] = 'Hello';
  object['greet'] = (name) => "${object['greeting']} $name";
  var message = object.callMethod('greet', ['JavaScript']);
  context['console'].callMethod('log', [message]);

Proxying and automatic conversion

When setting properties on a JsObject or passing arguments to a Javascript method or function, Dart objects are automatically converted or proxied to JavaScript objects. When accessing JavaScript properties, or when a Dart closure is invoked from JavaScript, the JavaScript objects are also converted to Dart.

Functions and closures are proxied in such a way that they are callable. A Dart closure assigned to a JavaScript property is proxied by a function in JavaScript. A JavaScript function accessed from Dart is proxied by a JsFunction, which has a apply method to invoke it.

The following types are transferred directly and not proxied:

  • "Basic" types: null, bool, num, String, DateTime
  • Blob
  • Event
  • HtmlCollection
  • ImageData
  • KeyRange
  • Node
  • NodeList
  • TypedData, including its subclasses like Int32List, but not ByteBuffer

  • Window

Converting collections with JsObject.jsify()

To create a JavaScript collection from a Dart collection use the JsObject.jsify constructor, which converts Dart Maps and Iterables into JavaScript Objects and Arrays.

The following expression creats a new JavaScript object with the properties a and b defined:

var jsMap = new JsObject.jsify({'a': 1, 'b': 2});

This expression creates a JavaScript array:

var jsArray = new JsObject.jsify([1, 2, 3]);



context JsObject

escapePrivateClassPrefix String

jsInterfaceTypes Iterable<ClassMirror>



addMemberHelper(MethodMirror declaration, String path, StringBuffer sb, {bool isStatic: false, String memberName}) → void

allowInterop(Function f) Function

Returns a wrapper around function f that can be called from JavaScript using the package:js Dart-JavaScript interop.

allowInteropCaptureThis(Function f) JSFunction

Returns a Function that when called from JavaScript captures its 'this' binding and calls f with the value of this passed as the first argument. When called from Dart, null will be passed as the first argument.

argsSafeForTypedInterop(Iterable args) → void

Check that that if any elements of args are Function it is safe to pass to JavaScript. To make a function safe, call allowInterop or allowInteropCaptureThis.

finalizeJsInterfaces() → void

Can be called to provide a predictable point where no more JS interfaces can be added. Creating an instance of JsObject will also automatically trigger all JsObjects to be finalized.

hasDomName(DeclarationMirror mirror) bool

registerJsInterfaces([List<Type> classes]) → void

Temporary method that we hope to remove at some point. This method should generally only be called by machine generated code.

safeForTypedInterop(arg) → dynamic

Check that that if arg is a Function it is safe to pass to JavaScript. To make a function safe, call allowInterop or allowInteropCaptureThis.

setupJsTypeCache() → void




A List proxying a JavaScript Array.



Proxies a JavaScript Function object.



Base class for all JS objects used through dart:html and typed JS interop.


Proxies a JavaScript object to Dart.