# Synchronous Platform.worker

**URL:** <https://discourse.elm-lang.org/t/synchronous-platform-worker/2009>\
**Category:** Learn\
**Created:** [September 19, 2018, 1:23pm UTC](https://discourse.elm-lang.org/t/synchronous-platform-worker/2009 "2018-09-19T13:23:07Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![ondrej](https://avatars.discourse-cdn.com/v4/letter/o/df705f/32.png) [@ondrej](https://discourse.elm-lang.org/u/ondrej)\
**Post date:** [September 19, 2018, 1:23pm UTC](https://discourse.elm-lang.org/t/synchronous-platform-worker/2009/1 "2018-09-19T13:23:07Z")

</div>

Hi, is there a way to use Platform.worker (or some alternative) to make a synchronous calls from javascript to Elm and back?

It would be useful for porting our js code if pure elm functions that don’t produce any tasks would be callable from js.

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<div class="post-metadata">

**Author:** ![rupert](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/rupert/32/1775_2.png) [@rupert](https://discourse.elm-lang.org/u/rupert)\
**Post date:** [September 21, 2018, 2:10pm UTC](https://discourse.elm-lang.org/t/synchronous-platform-worker/2009/2 "2018-09-21T14:10:53Z")

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I have been looking into this today, and I think I can see how it is done, but I am not sure if it will be synchronous. I wrote “hello world” Platform.worker style:

### Example

```auto
main : Program String String Never
main =
    Platform.worker
        { init = \flags -> ( "Hello " ++ flags, Cmd.none )
        , update = \msg model -> ( model, Cmd.none )
        , subscriptions = \model -> Sub.none
        }

```

### Structure of the Javascript

Looking at the generated code, it is structure is as below (I’ve added some whitespace to try and show better how it is structured):

```auto
(function(scope)
{

  ...
  function _Platform_export(exports)
  {
  	scope['Elm']
		? _Platform_mergeExportsProd(scope['Elm'], exports)
		: scope['Elm'] = exports;
  }

  ...
  var author$project$Main$main = elm$core$Platform$worker(
  {
      ....
  });

  _Platform_export({'Main':{'init':author$project$Main$main(elm$json$Json$Decode$string)(0)}});
 
} (this) );

```

In summary, an anonymous function is immediately called, and an internal function within that sets the value of `Main` in the `Elm` scope, this has a field called `init` which is the worker. None of the functions you wrote are exposed outside of this big anonymous function that is wrapped around everything.

### Passing args in: Running the Worker

You can run the worker like this ([https://guide.elm-lang.org/interop/flags.html](https://guide.elm-lang.org/interop/flags.html)):

```auto
var app = Elm.Main.init({
  flags: "ondrej"
});

```

But there does not appear to be a way to get hold of the `model` from inside the Elm program?

### Getting data out: ports?

You set up ports on the worker like this ([https://guide.elm-lang.org/interop/ports.html](https://guide.elm-lang.org/interop/ports.html)):

In the Elm program:

```auto
port sayHello : String -> Cmd msg

main : Program String String Never
main =
    Platform.worker
        { init = \flags -> ( "Hello " ++ flags, sayHello <| "Hello " ++ flags)
        , update = \msg model -> ( model, Cmd.none )
        , subscriptions = \model -> Sub.none
        }

```

In javascript:

```auto
function greet(name) {
  var greeting;

  var app = Elm.Main.init({
    flags: name
  });

  app.ports.subscribe(function(data) {
    greeting = data;
  });

  return greeting;
}

greet('ondrej');

```

I am not sure though, that the first command returned from `init` will be processed synchronously to make this work?

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<div class="post-metadata">

**Author:** ![ondrej](https://avatars.discourse-cdn.com/v4/letter/o/df705f/32.png) [@ondrej](https://discourse.elm-lang.org/u/ondrej)\
**Post date:** [September 27, 2018, 6:08am UTC](https://discourse.elm-lang.org/t/synchronous-platform-worker/2009/3 "2018-09-27T06:08:24Z")

</div>

It looks like it doesn’t work with `Platform.worker` [https://ellie-app.com/3svkC5KgR6na1](https://ellie-app.com/3svkC5KgR6na1) but with `Browser.element` it does [https://ellie-app.com/3svnmFcvNRXa1](https://ellie-app.com/3svnmFcvNRXa1).

I’m not sure exactly why 🤔

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<div class="post-metadata">

**Author:** ![system](https://canada1.discourse-cdn.com/flex035/uploads/elm_lang/original/1X/50a05e53677a2c3b47776d7abd0f113eb50193a1.png) [@system](https://discourse.elm-lang.org/u/system)\
**Post date:** [October 7, 2018, 6:08am UTC](https://discourse.elm-lang.org/t/synchronous-platform-worker/2009/4 "2018-10-07T06:08:32Z")

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