A singleton isolated worker for all platforms. On most platforms, it uses Flutter's Isolate, except on the web, since Isolate is not available, it uses Worker instead.


  • 💙 Easy to use*
  • 👬 Identical syntax to the compute function provided by Flutter*
  • 🚫 Not a one-off worker
  • 🌐 Available on web**

*except on web **by using JsIsolatedWorker


Basic example

int doSomeHeavyCalculation(int count) {
    // ...
void main() {
    // if using compute function:
    // compute(doSomeHeavyCalculation, 1000);
    IsolatedWorker().run(doSomeHeavyCalculation, 1000);

If we want to do some heavy work that does not need any arguments and/or return values.

void doSomethingHeavy() {
    // ...

void doSomethingHeavy(void _) {
    // ...

void main() {
    IsolatedWorker().run(doSomethingHeavy, null);

Web example

We can utilize the JsIsolatedWorker for spawning a web worker. However, it cannot use Dart closures as messages to the worker because of some limitations (I have tried using JSONfn and allowInterop but no luck). Instead we need to use native JS closures. In order to do this, we can utilize existing JS APIs or by importing external libraries/files.

Let's assume we want to stringify objects using JSON.stringify.

void main() {
        functionName: ['JSON', 'stringify'],
        arguments: {},
        // optional argument, in case web worker is not available.
        fallback: () {
            return '{}';
    // prints "{}"

Now let's assume we have external js libraries/files that we want the worker to use.

void main() async {
    // import the scripts first
    // and check if web worker is available
    final bool loaded = await JsIsolatedWorker().importScripts(['myModule1.js']);
    // web worker is available
    if(loaded) {
        print(await JsIsolatedWorker().run(
            functionName: 'myFunction1',
            arguments: 'Hello from Dart :)',
        print('Web worker is not available :(');

Shared Platform Worker

There might be a case where we need to use both IsolatedWorker and JsIsolatedWorker. There are many ways to go about it. We can use conditional import, dependency injections, or WorkerDelegator.


For example, we have a foo Dart method that returns a list of numbers from 1 to count.

List<int> foo(int count) {
    List<int> result = <int>[];
    for(int i = 1; i <= count; i++) {
    return result;

And a JS file foo.js containing a foo JS method that does the same thing.

function foo(count) {
    let result = [];
    for(let i = 1; i <= count; i++) {
    return result;

To call these methods using WorkerDelegator, we need to create a DefaultDelegate, JsDelegate, and WorkerDelegate first. Then register the WorkerDelegate to our WorkerDelegator.

Future<void> main() async {
    const DefaultDelegate<int, List<int>> fooDelegate = 
        DefaultDelegate(callback: foo);
    const JsDelegate fooJsDelegate = JsDelegate(callback: 'foo');

    const WorkerDelegate<int, List<int>> workerDelegate = 
            key: 'foo',
            defaultDelegate: fooDelegate,
            jsDelegate: fooJsDelegate,

    // Don't forget to call importScripts for our "foo" js method.
    await WorkerDelegator().importScripts(const <String>['foo.js']);

After registering our WorkerDelegate, we just need to call it using its key "foo".

print(await WorkerDelegator().run('foo', 9));

The key can be anything except null.

enum DelegateKeys { foo, bar }
const WorkerDelegate<int, List<int>> workerDelegate = 
        key: DelegateKeys.foo,
        defaultDelegate: fooDelegate,
        jsDelegate: fooJsDelegate,

Notes: WorkerDelegator() is a singleton. If we need to create a new instance, we can create one using

    delegates: /* pass our WorkerDelegates here. OPTIONAL. */

Web Worker Setup

In order for JsIsolatedWorker to run properly on Flutter web, there needs to be a single worker.js file in the web folder. You can download it here and put it in your web folder like below

web /


  • Fetch - an example of how to get response from a URL using IsolatedWorker and JsIsolatedWorker