Suppose we'd like to invoke the JavaScript function JSON.stringify which receives an object, encodes it into a JSON string and returns it.

All we'd have to do is write the function signature, mark it as external and annotate it with the @JS annotation:

 @JS("JSON.stringify")
external String stringify(obj);

The @JS annotation will be used from here on out to mark Dart classes that we'd like to use in JavaScript as well.

Suppose we'd like to wrap the Google Maps JavaScript API google.maps:

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

import "package:js/js.dart";

@JS()
class Map {
  external Map(Location location);
  external Location getLocation();
}

We now have the Map Dart class which corresponds to the JavaScript google.maps.Map class.

Running new Map(someLocation) in Dart will invoke new google.maps.Map(location) in JavaScript.

Note that you don't have to name your Dart class the same as the JavaScript class:

@JS("LatLng")
class Location {
  external Location(num lat, num lng);
}

The Location Dart class corresponds to the google.maps.LatLng class.

Using inconsistent names is discouraged as they can create confusion.

It's common practice in JavaScript to pass object literals to functions:

// JavaScript
printOptions({responsive: true});
Unfortunately we cannot pass Dart Map objects to JavaScript in these cases.

What we have to do is create a Dart object that represents the object literal and contains all of its fields:

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

  external factory Options({bool responsive});
}

Note that the Options Dart class doesn't correspond to any JavaScript class. As such we must mark it with the @anonymous annotation.

Now we can create a stub for the original printOptions function and call it with a new Options object:

// Dart
@JS()
external printOptions(Options options);

printOptions(new Options(responsive: true));