March 18, 2021

#Mithril.js + esbuild + JSX

EDIT 7/18/2022: esbuild has since added support for string literals as JSX fragments, meaning much of what I wrote below is now unnecessary. To use JSX with Mithril and esbuild, all you need to do is set jsxFragment: '"["' in your esbuild config. See here for an example.

I recently bootstrapped a Mithril.js project using esbuild for my bundling purposes. If you don't already know, esbuild is a next-gen bundler written in Go by Evan Wallace that is magnitudes faster than rollup, webpack, or parcel (all of which are written in JS/TS).

esbuild comes with JSX support out of the box. While I don't normally use JSX, I decided to create a demo for showing off that JSX works just fine in a Mithril application. However, I ran into one major issue -- esbuild wasn't compiling JSX fragments correctly when providing m.fragment as the JSX fragment pragma.

After some digging around, I found this comment by Claudia Meadows.

In short, there's no support for Mithril fragments as Components currently. esbuild (as well as transpilers like Sucrase) compiles JSX using the provided jsx pragma + jsx fragment method in the form of:

React.createElement(React.Fragment, null, "Stuff");

See here for more details.

Which in Mithril's case, can't work with m.fragment. Following Claudia's advice, defining a simple Fragment component as so worked for me. I just put this in my index.js file near the top:

m.Fragment = { view: vnode => vnode.children };

Then I just set jsxFragment to m.Fragment instead of m.fragment in my esbuild config and voilà! Fragments work as you'd expect.

var JSX = {
    view: function() {
        return (
                <p>You could use JSX with Mithril.js as well.</p>
                <p>Be aware that this requires a build-step.</p>
                <p style={{ color: 'red' }}>
                    Attributes work as expected.
                <p>This application uses <a href="">esbuild</a> to convert JSX.</p>

For reference, you can see my full esbuild config and my bundle scripts here. Direct any questions to and I'd be happy to answer them.