Elm: building Native modules

29 Oct 2014

I couldn’t find any documentation on how to write modules written in pure JavaScript for Elm so I took a stab at it myself, using elm-d3 and the Elm compiler as a reference. Here’s a quick write-up.

It’s important to note that doing this is not encouraged by Evan (the Elm creator). It’s a hack and subject to compiler implementation changes. Use at your own risk.

The problem

Elm is great but there are functions that aren’t implemented as standard, aren’t worth the effort to write in Elm itself, and are better built in JavaScript.

While Elm has a JavaScript interoperability pattern called ports, they’re async-only and have a couple of other unfortunate constraints so aren’t ideal for some use-cases.

Sometimes you just gotta call a function.

The solution

A quick bit of background to begin…

Elm imports look like this:

import SomeModule
import AnotherModule ( specific, functions )
import AThirdModule ( .. ) -- all the things
import Native.Logger

At compile time the compiler goes out to find the modules you import (SomeModule maps to a file of the same name with a .elm extension) unless the module name has the prefix Native..

If the .elm files aren’t there, it will blow up — this is good. If the name contains Native., the Elm compiler will continue even if the file doesn’t exist.

Assuming the files are present it compiles the importing code to something like:

var $Some$Module = Elm.SomeModule.make(_elm),
    $Another$Module = Elm.AnotherModule.make(_elm),
    $A$Third$Module = Elm.AThirdModule.make(_elm),
    $Native$Logger = Elm.Native.Logger.make(_elm);

You can see that Elm assumes modules are there at runtime, and that it calls a make method to initialise the module. To build a native module, we just need to add something to the Elm.Native namespace before the main app loads (but after the runtime loads).

So, here’s an example native module called Logger:

Elm.Native.Logger = {};
Elm.Native.Logger.make = function(elm) {
    elm.Native = elm.Native || {};
    elm.Native.Logger = elm.Native.Logger || {};
    if (elm.Native.Logger.values) return elm.Native.Logger.values;
    return elm.Native.Logger.values = {
        log: function (x) {
            return x;

You can include this in the page in any way you like, so long as it comes after the Elm runtime:

<meta charset="UTF-8">
<title>Your App</title>

<!-- Runtime -->
<script src="elm-runtime.js"></script>
<!-- Native files -->
<script src="Native/Logger.js"></script>
<!-- Compiled Elm bundle -->
<script src="build/App.js"></script>
window.onload = function () {

As usual, in production, concatenate and minify those files!

To finish up, here’s a little snippet for generating these modules:

function ElmNativeModule(name, values) {
    Elm.Native[name] = {};
    Elm.Native[name].make = function(elm) {
        elm.Native = elm.Native || {};
        elm.Native[name] = elm.Native[name] || {};
        if (elm.Native[name].values) return elm.Native[name].values;
        return elm.Native[name].values = values;

Use it like this:

ElmNativeModule('Logger', {
    log: function (x) {
        return x;

Remember, this is not recommended. Try to use ports first, and prepare for this to break when the compiler implementation changes.