Elm Outgoing Port Message Delivery Without a JavaScript Subscriber

Direct Answers

• Buffering: No. Elm 0.19.1 does not buffer outgoing port messages. If no JavaScript subscriber is registered at the moment port myPort : String -> Cmd msg is executed, the value is dropped immediately.
• Detection: No. The Elm runtime provides no callback, return value, or error signal when a message is dropped. The Cmd always completes successfully from Elm's perspective.
• Compiler enforcement: No. There is no port annotation, pragma, or compiler flag that requires a JavaScript subscription before sending. The compiler only checks type signatures.

Why This Happens

Elm treats outgoing ports as fire-and-forget asynchronous boundaries. The compiled JavaScript roughly implements:

app.ports.myPort = { subscribers: [], send: function(value) { this.subscribers.forEach(fn => fn(value)); } }

When subscribers is empty, the forEach loop simply does nothing—no error, no queue, no retry. This is intentional: Elm avoids coupling the Elm runtime to host-side lifecycle.

Practical Mitigation Patterns

1. Handshake via Incoming Port (Recommended)

Have JavaScript signal readiness through an incoming port before Elm sends any outgoing messages.

-- Elm
port outgoing : String -> Cmd msg
port ready : (() -> msg) -> Sub msg

init = (Model [], Cmd.none)
subscriptions model = ready Ready

update msg model =
  case msg of
    Ready -> (model, outgoing "hello")
    _ -> (model, Cmd.none)

// JavaScript
const app = Elm.Main.init({ node: document.getElementById('app') });
app.ports.outgoing.subscribe(console.log);
app.ports.ready.send(null); // Tell Elm we're listening

2. Queue in Elm State Until Ready

If you cannot control initialization order, accumulate messages in a List String and flush when the ready signal arrives.

3. Shared Flag in localStorage (Last Resort)

Write a flag from JavaScript on subscribe; read it from Elm via a one-time incoming port or flags at startup. Less reliable across page reloads.

Verification Steps

1. Create a minimal Elm app with an outgoing port.
2. Compile with elm make src/Main.elm --output=elm.js.
3. Embed in HTML without calling app.ports.myPort.subscribe.
4. Trigger a port send from init or update.
5. Observe: no console output, no Elm error, message disappears.
6. Add app.ports.myPort.subscribe(msg => console.log('received', msg)) before Elm.Main.init; verify messages arrive.

One Diagnostic Detail That Changes the Recommendation

If you are using a framework (Electron, Tauri, Capacitor, etc.) that injects its own port wiring, confirm whether it auto-subscribes before your Elm code runs. In that case, the handshake may be unnecessary—but only if the framework guarantees subscription before init completes.