For article on mattharris.tech

main.dart

// Import the JS interop library
import 'dart:js';

// export the js wrapper as the function you want, since I'm only exporting one, i used the handy => notation.
void main() => context['adder'] = jsAdder;

// This wraps adder, returning the object we created, now I made it return a map, since that's usually more useful.
Object jsAdder(num a,num b) {
  return new JsObject.jsify(adder(a, b));
}

// This pure dart function, called, adder, takes two numbers and returns their result in an object.
Map<String, num> adder(num a, num b) {
  Map<String, num> output = {
   'result': a+b 
  }
  return output;
}