An example of a JavaScript library that fetches your contacts from an "API".

fetch_contacts.dart

@JS()
library fetch_contacts_interop;

import 'package:func/func.dart';
import 'package:js/js.dart';

// Natively calls "fetch_contacts", but we can use typed Dart code while developing!
@JS('fetch_contacts')
external void fetchContacts(String prefix, VoidFunc1<List<String>> callback);

// Example use
void main() {
  fetchContacts('A', (results) => 'Contacts: $results'));
}

fetch_contacts.js

// An example of a JavaScript library that fetches your contacts from an "API".
//
// Example use:
// fetch_contacts('C', (contacts) => { ... });

const contacts = [
  'Amy',
  'Beth',
  'Carl',
  'Dave'
];

function fetch_contacts(prefix, callback) {
  let results = contacts.filter((contact) => {
    return contact.startsWith(prefix);
  });
  setTimeout(() => {
    callback(results);
  }, 0);
}