JSX In Web Components


How'd I Get There

So as part of the recent rewrite of this site I decided to experiment with Web Components. Or custom elements specifically. I had used Web Components at a previous job, so they were not new to me, but I had not used them much personally.

One decision I needed to make first is how to generate html. I will tell you briefly what I tried, before I settled on JSX, and then I will explain briefly how to do it.

First I tried tagged template literals. Going back as far as 2015 I have used them and for a long time I had a framework that was very small — less than 3kb — but for various reasons I wanted to see if I could go even more minimal. Also lit is more code than my own framework, so that was not a contender.

Second I tried Declarative Shadow DOM. This was actually great. The problem I ran into with it was that to my surprise Declarative Shadow DOM works without Javascript. That does make sense I guess, but it means that combined with sparse support you have to support a matrix of JS, no JS, support, and no support for styling. I may write more about that at some point, because I will likely come back to Declarative Shadow DOM for limited things. For instance the code blocks on this site perhaps where it makes sense to do the syntax highlighting on the server, but I also want some interactivity like toggling word wrapping which I had in this site's previous iteration and have removed for this one.

So eventually I settled on the idea of using JSX. I should say right up front that I have never been a fan of JSX. It's seemed unnecessary. We have template literals after all, which have the advantage of being able to be cached where as JSX is just function calls on every render. But JSX has a few advantages. First support for them is built into everything basically: Babel, and more importantly for me, SWC. Second, code editor support is great. It means I do not have to install a plugin in VS Code just for syntax highlighting. A possible third advantage to JSX is that I work with React at work now, so staying in that realm for hobby stuff helps prevent context switching.

How To

OK so how do we use JSX with Web Components? There are really just two steps with an optional third.

  1. Enable JSX compiling in whatever you are using. Babel, etc.
  2. Define an h function.
  3. Define a Fragment function if you use fragments.

For step 1, if you are using Babel you can use their JSX plugin along with setting runtime to classic and pragma to h and pragmaFrag to Fragment.

And here is an example of h and Fragment. You just need to be sure to import them or make sure they are defined wherever you use JSX.

function h(tag, props, ...children) {
  children = children.flat(Infinity);

  if (typeof tag === "function") return tag({...props, children});

  let node = document.createElement(tag);

  for (let [key, val] of Object.entries(props ?? {})) {
    if (key.startsWith("on")) {
      node.addEventListener(key.substring(2).toLowerCase(), ...[].concat(val));
    } else {
      node[key] = val


  return node;

function Fragment({children}) {
  return children;

My dream is the have a html tagged template function built into browsers, but in the meantime as long as I'm using Babel, or similar, JSX is a pretty good substitute