Browser/user language in pure Elm app


Hi! :wave:

I have a small app where I’m trying to develop exclusively with elm reactor and elm make —output=index.html

The application has language translations and localization of numbers, and I would like to choose the default language when booting the app to match the browser language of the user.

In JS, this information is in navigator.language:

Is there any way to get this language with pure Elm?

(I know how to do it with ports and a custom HTML, but I want to keep using elm-reactor)

Thank you, and have a nice day!


Unfortunately no. You’re stuck if you want to keep the reactor. There isn’t a way in the reactor to get the compiled javascript as a url, you only have the compiled to html version available, but… If you want to keep elm-reactor for its simpilicity, you can have a look at parcel: it’s a zero config bundler. Just create a index.html and index.js (that import Main.elm). Then you can use ports & all…

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There are two tricks that I can think of:

  1. If you have a server to talk to, you could make an HTTP request to it on init and have it return to you your Accept-Language header. You could also use if you trust it as it accepts CORS requests. For example

  2. Decode an event that alllows you to access window.navigator.language, for example using the first mouse move: (you should remove the subscription once you have the language though).
    You could also ask the user to press a key or click a button on init and get it the same way (maybe using a nice welcome banner as a pretext).

subscriptions : Model -> Sub Msg
subscriptions model =
    Events.onMouseMove ( GotLang decodeLang)

decodeLang : Decode.Decoder String
decodeLang = [ "view", "navigator", "language" ] Decode.string

At last note that some browsers differentiate the browser interface language from pages content language, so the two techniques may give different results.


Thanks @dmy, very useful.

I’ve made a little example by manually triggering focus, not requiring user interaction: A small issue is that the app focus is shown, which you could hide with CSS outline or trigger a blur from Elm. All solvable.

This is a very interesting approach, it could probably be used for accessing all kinds of useful information in navigator, like navigator.onLine.

In any case, for a more serious app it is quite clear that Flags and ports would be the easy way to go for this.

There is probably a way to have an entry point for compiling with a custom HTML, and another file with defaults for loading the app with elm reactor. That could work for flags information.

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I can recommend having a look at elm-live as a replacement for reactor. It can use custom html.

I’ve used it in a project of mine. You can have a look at my npm start script. start:no-sw stands for “no service worker”, since sometimes I want to test the app without caching or other SW features.


If you are down to building the compiler yourself, we extended the reactor to do exactly what you need:

We added two flags: a js and a debug flag. Getting the js in debug mode from the reactor would then look like this: http://localhost:8000/src/Main.elm?output=js&debug=true

You can find the extened compiler with build instructions here and an example here

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